drm/ast: Declare helpers for POST in header

Provide POST helpers in header file before splitting up the AST
POST code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-2-tzimmermann@suse.de