drm/i915: Flush primary plane changes in sprite code

Flush the primary plane changes when enabling/disabling the primary
plane in response to sprite visibility.

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