drm: rcar-du: Disable alpha blending for DU planes used with VSP

When the input to a DU channel comes from a VSP, the DU doesn't perform
any blending operation. Select XRGB8888 instead of ARGB8888 to ensure
that the corresponding registers don't get written with invalid values.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>