drm/ast: Merge config and chip detection

Detection of the configuration mode and the chipset model are
linked to each other. One uses values from the other; namely the
PCI device revision and the SCU revision. Merge this code into
a single function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com> # AST2600
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Link: https://patchwork.freedesktop.org/patch/msgid/20230621130032.3568-15-tzimmermann@suse.de