drm/i915: Make primary_enabled match the actual hardware state

The BIOS can enable a pipe but leave the primary plane disabled. This
coflicts with out current idea of primary_enabled. Read the actual
hardware plane state and set primary_enabled appropriately.

We currently assume that primary_enabled is always true when we're about
to disable a crtc. That needs to change now as the plane may not be
enabled. So replace the relevant WARNs with early returns in
intel_{enable,disable}_primary_hw_plane().

Fixes the following warning
[    3.831602] WARNING: CPU: 0 PID: 1112 at linux/drivers/gpu/drm/i915/intel_display.c:1918 intel_disable_primary_hw_plane+0xe4/0xf0 [i915]()

which got introduced here by me:
 commit e9e39655c0c30cddc3f8c09a757678a24dd36737
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Mon Apr 28 15:53:25 2014 +0300

    drm/i915: Remove useless checks from primary enable/disable

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>