drm/i915: Wait for vblank between disabling a sprite and unpinning the fb

When disabling a sprite, wait for the sprite to stop fetching data
from memory before unpinning the fb.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>