drm/ast: Refactor ast_post_gpu() by Gen

Reorganize ast_post_gpu() so that it first branches by Gen and then
by config mode and TX chip. This will later make it possible to split
up the function by Gen.

The helper ast_init_3rdtx() only handles Gen4 and Gen5, so leave it
out from the other Gens.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250117103450.28692-4-tzimmermann@suse.de