drm/ast: Move Gen7+ POST code to separate source file

Move POST code for Gen7+ to separate source file and hide it in
ast_2600_post(). There's not much going on here except for enabling
the DP transmitter chip.

v2:
- simplify logic (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-3-tzimmermann@suse.de