drm/udl: Add constants for display-mode registers

Add constants for the registers the contain various display-mode
parameters and update the mode-setting function. 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/20221006095355.23579-13-tzimmermann@suse.de