[media] drm: rcar-du: Add alpha support for VSP planes

Make the global alpha multiplier of VSP planes configurable through the
alpha property, exactly as for the native DU planes.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>