drm/ast: Move Gen4+ POST code to separate source file

Move POST code for Gen4+ to separate source file and hide it in
ast_2300_post(). With P2A configuration, it performs a full board
POST and enables the transmitter chip; otherwise it only enables the
transmitter chip.

Also fix coding style in several places. 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/20250706162816.211552-5-tzimmermann@suse.de