drm:rcar-du: Enable ABGR and XBGR formats

These formats are used by Android so having them available
allows the DU to be used for composition operations.

Signed-off-by: Damian Hobson-Garcia <dhobsong@igel.co.jp>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>