drm/i915: Make wait_for_flips interruptible.

Move it from intel_crtc_atomic_commit to prepare_plane_fb.
Waiting is done before committing, otherwise it's too late
to undo the changes.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ander Conselvan De Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>