drm/udl: Move register constants to udl_proto.h

Move the existing register constants to a new file in preparation of
adding more of them. Renaming is intentional. 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/20221006095355.23579-12-tzimmermann@suse.de