drm/i915: Eliminate plane control register RMW from sprite code

Replace the RMW access with explicit initialization of the entire plane
control register, as was done for primary planes in:

 commit f45651ba
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Fri Aug 8 21:51:10 2014 +0300

    drm/i915: Eliminate rmw from .update_primary_plane()

The automagic primary plane disable is still doing RMWs, but that will
require more work to untangle, so leave it alone for now.

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