sna/video/sprite: Query planes for RGB565

Not all sprite planes support RGB565. Insrtead of hardcoding which
platforms have it let's ask the kernel instead.

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>