drm/ast: Move struct ast_dramstruct to ast_post.h

Declare struct ast_dramstruct in ast_post.h and remove its original
header file.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250706162816.211552-7-tzimmermann@suse.de