drm/i915: Always call intel_update_sprite_watermarks() when disabling a plane

ILK and VLV codepaths didn't update sprite watermarks when disabling a
sprite. Make them do that.

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>