drm/ast: Move detection code into PCI probe helper

Detect device type and config mode in the PCI probe helper, but leave
DRM device initialization where it is. Structures the driver probe and
setup code into a detection and an initialization phase.

A later patch can add branching to the device-initialization code. Each
chip type can have it own initializer function, if necessary.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231116100240.22975-11-tzimmermann@suse.de