drm/udl: Add register constants for color depth

Add the register constants for setting the color depth. The driver
only uses 16bpp. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-14-tzimmermann@suse.de