drm/udl: Add register constants for framebuffer scanout addresses

Add register constants for the framebuffer scanout addresses and
update the related helper functions. No functional changes.

v2:
	* extract address bytes with helper macros (Javier)
	* fix comments

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-16-tzimmermann@suse.de