drm/gud: Add GUD_PIXEL_FORMAT_R8

Add support for 8-bit greyscale format.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929191201.34456-5-noralf@tronnes.org