drm/i915: Use skl_plane_ctl() for the SKL "sprite" planes

On SKL the planes are uniform so the "sprites" can use the
primary plane code perfectly fine. The only difference we
have is the color key handling, but since we never enable that
for the primary plane the same code works just fine.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170317211808.14693-3-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>