sna/video/sprite: Add sprite planes in order

On SKL+ dst color keying only works between the first sprite and the
primary. We probably wante the first Xv port to be the first sprite
plane so that the user gets working colorkeying for the port that is
most likely to be used first. No way to get dst colorkeying with the
other ports :(

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>