drm/ast: Move Gen6+ POST code to separate source file

Move POST code for Gen6+ to separate source file and hide it in
ast_2500_post(). With P2A configuration, it performs a full board
POST; otherwise it enables the transmitter chip. No changes to the
overall logic.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-4-tzimmermann@suse.de