drm/i915: Don't disable primary when color keying is used

When color keying is used, the primary may not be invisible even though
the sprite fully covers it. So check for color keying before deciding to
disable the primary plane.

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