drm: rcar-du: Add support for missing 16-bit RGB1555 formats

Add support for the DRM_FORMAT_RGBA5551, DRM_FORMAT_RGBX5551,
DRM_FORMAT_ABGR1555, DRM_FORMAT_XBGR1555, DRM_FORMAT_BGRA5551 and
DRM_FORMAT_BGRX5551 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>