drm: rcar-du: Add support for missing 16-bit RGB4444 formats

Add support for the DRM_FORMAT_RGBA4444, DRM_FORMAT_RGBX4444,
DRM_FORMAT_ABGR4444, DRM_FORMAT_XBGR4444, DRM_FORMAT_BGRA4444 and
DRM_FORMAT_BGRX4444 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>