drm/i915: Clear intel_crtc->atomic before updating it.

If an atomic update fails intel_crtc->atomic may have have some values left
from the last atomic check. One example is atomic->wait_for_vblank,
which results in spurious errors in kms_flip.

[ 1551.892708] ------------[ cut here ]------------
[ 1551.892721] WARNING: CPU: 3 PID: 4179 at ../drivers/gpu/drm/drm_irq.c:1199 drm_wait_one_vblank+0x197/0x1a0 [drm]()
[ 1551.892722] vblank not available on crtc 2, ret=-22
[ 1551.892751] Modules linked in: snd_hda_intel i915 drm_kms_helper drm
intel_gtt i2c_algo_bit cfbfillrect syscopyarea cfbimgblt sysfillrect
sysimgblt fb_sys_fops cfbcopyarea agpgart cfg80211 binfmt_misc
snd_hda_codec_hdmi intel_rapl iosf_mbi x86_pkg_temp_thermal coretemp
kvm_intel snd_hda_codec_realtek kvm snd_hda_codec_generic iTCO_wdt
aesni_intel aes_x86_64 glue_helper lrw snd_hda_codec gf128mul
ablk_helper cryptd snd_hwdep psmouse snd_hda_core pcspkr snd_pcm
snd_timer snd lpc_ich i2c_i801 mfd_core soundcore wmi evdev [last
unloaded: drm]
[ 1551.892753] CPU: 3 PID: 4179 Comm: kms_pipe_crc_ba Tainted: G     U  W       4.3.0-reg+ #6
[ 1551.892754] Hardware name:                  /DZ77BH-55K, BIOS BHZ7710H.86A.0100.2013.0517.0942 05/17/2013
[ 1551.892758]  ffffffffa03128d8 ffff8800cec73890 ffffffff812c0f3c ffff8800cec738d8
[ 1551.892760]  ffff8800cec738c8 ffffffff8104ff36 ffff880116ae2290 0000000000000002
[ 1551.892762]  ffff8800d39fcda0 ffff8800d038b4d0 ffff8800d42b5550 ffff8800cec73928
[ 1551.892763] Call Trace:
[ 1551.892768]  [<ffffffff812c0f3c>] dump_stack+0x4e/0x82
[ 1551.892771]  [<ffffffff8104ff36>] warn_slowpath_common+0x86/0xc0
[ 1551.892773]  [<ffffffff8104ffbc>] warn_slowpath_fmt+0x4c/0x50
[ 1551.892781]  [<ffffffffa02e6708>] ? drm_vblank_get+0x78/0xd0 [drm]
[ 1551.892787]  [<ffffffffa02e6d47>] drm_wait_one_vblank+0x197/0x1a0 [drm]
[ 1551.892813]  [<ffffffffa03d052f>] intel_post_plane_update+0xef/0x120 [i915]
[ 1551.892832]  [<ffffffffa03d11d2>] intel_atomic_commit+0x4c2/0x1600 [i915]
[ 1551.892862]  [<ffffffffa02ff0c7>] ? drm_atomic_check_only+0x147/0x5e0 [drm]
[ 1551.892872]  [<ffffffffa02feeb7>] ? drm_atomic_add_affected_connectors+0x27/0xf0 [drm]
[ 1551.892881]  [<ffffffffa02ff597>] drm_atomic_commit+0x37/0x60 [drm]
[ 1551.892887]  [<ffffffffa034301a>] restore_fbdev_mode+0x28a/0x2c0 [drm_kms_helper]
[ 1551.892895]  [<ffffffffa0345253>] drm_fb_helper_restore_fbdev_mode_unlocked+0x33/0x80 [drm_kms_helper]
[ 1551.892900]  [<ffffffffa03452cd>] drm_fb_helper_set_par+0x2d/0x50 [drm_kms_helper]
[ 1551.892920]  [<ffffffffa03e7a9a>] intel_fbdev_set_par+0x1a/0x60 [i915]
[ 1551.892923]  [<ffffffff8131a5a7>] fb_set_var+0x1a7/0x3f0
[ 1551.892927]  [<ffffffff8109732f>] ? trace_hardirqs_on_caller+0x12f/0x1c0
[ 1551.892931]  [<ffffffff81314f32>] fbcon_blank+0x212/0x2f0
[ 1551.892935]  [<ffffffff81373f4a>] do_unblank_screen+0xba/0x1d0
[ 1551.892937]  [<ffffffff8136b725>] vt_ioctl+0x13d5/0x1450
[ 1551.892940]  [<ffffffff8107cdd1>] ? preempt_count_sub+0x41/0x50
[ 1551.892943]  [<ffffffff8135d8a3>] tty_ioctl+0x423/0xe30
[ 1551.892947]  [<ffffffff8119f721>] do_vfs_ioctl+0x301/0x560
[ 1551.892949]  [<ffffffff8119b1e3>] ? putname+0x53/0x60
[ 1551.892952]  [<ffffffff811ab376>] ? __fget_light+0x66/0x90
[ 1551.892955]  [<ffffffff8119f9f9>] SyS_ioctl+0x79/0x90
[ 1551.892958]  [<ffffffff81552e97>] entry_SYSCALL_64_fastpath+0x12/0x6f
[ 1551.892961] ---[ end trace 3e764d4b6628c91c ]---

Testcase: kms_flip
Reported-and-tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org #v4.3
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/5649C2BA.6080300@mblankhorst.nl