drm/ast: Add enum ast_config_mode

The config mode used to be a field in struct ast_device. Turn it into
a named type. We'll need this for device detection.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116100240.22975-9-tzimmermann@suse.de