sna/video/sprite: Add NV12 support

Starting from ~KBL planes can do NV12. Let's make use that
capability in the sprite Xv adaptor.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>