drm/gud: Add GUD_PIXEL_FORMAT_RGB888

Add support for the RGB888 pixel format.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929191201.34456-7-noralf@tronnes.org