drm/ast: Align naming in widescreen detection code to manual

Rename variables and register constants to align with the programming
manual. Add new constants where necessary.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131092257.115596-3-tzimmermann@suse.de