drm: shmobile: Add support for DRM_FORMAT_XRGB8888

DRM_FORMAT_XRGB8888 aka XR24 is the modus francus of DRM, and should be
supported by all drivers.

The handling for DRM_FORMAT_XRGB8888 is similar to DRM_FORMAT_ARGB8888,
just ignore the alpha channel.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/888cde6d1f70f1610931c4a89801060bb6765ff2.1684854992.git.geert+renesas@glider.be