drm/i915: Kill the leftover RMW from ivb_sprite_disable()

We still had one lingering RMW in ivb_sprite_disable(), all the other
RMWs were killed off from the sprite code some time ago. Kill the
straggler too.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>