drm/ast: Partially implement POST without ast device instance

We'll have to do some of the GPU POSTing for detecting the ast device
type. Make this work without an instance of the ast device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116100240.22975-8-tzimmermann@suse.de