drm/ast: Gen1: Fix open-coded register access

drm/ast: Gen1: Fix open-coded register access

Replace all open-coded access to MCR registers in Gen1 with the
appropriate calls to ast_moutdwm() and ast_mindwm(). Use MCR register
constants.

For the poll loop on MCR100, add ast_moutdwm_poll(). The helper polls
the register until it has been updated to the given value. Relax the
CPU while busy-waiting.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260327133532.79696-11-tzimmermann@suse.de