drm/fourcc: Add R8 to drm_format_info

Add an entry in drm_format_info for the existing format DRM_FORMAT_R8.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210929191201.34456-2-noralf@tronnes.org