drm/ast: Move register constants to ast_reg.h

Improve readability by putting all register constants into a separate
header file. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231017083653.10063-13-tzimmermann@suse.de