drm/i915: Remove double wait_for_vblank on broadwell.

wait_vblank is already set in intel_plane_atomic_calc_changes
for broadwell, waiting for a double vblank is overkill.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1447945645-32005-5-git-send-email-maarten.lankhorst@linux.intel.com