drm: rcar-du: Add support for missing 32-bit RGB formats

Add support for the DRM_FORMAT_RGBA8888, DRM_FORMAT_RGBX8888,
DRM_FORMAT_ABGR8888 and DRM_FORMAT_XBGR8888 formats to the DU driver.
Those formats are only available on Gen3.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>