drm/udl: Add constants for commands

Add constants for the various commands that the driver can send to
the device and update the respective helper functions. 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/20221006095355.23579-17-tzimmermann@suse.de