drm/ast: Gen2: Fix open-coded register access

drm/ast: Gen2: Fix open-coded register access

Replace all open-coded access to MCR and SCU registers in Gen2 with the
appropriate calls to ast_moutdwm() and ast_mindwm(). Use MCR and SCU
register constants. Name variables according to registers.

The values in MCR04 that control VRAM allocation do not look correct.
Leave a FIXME comment for later investigation.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patch.msgid.link/20260327133532.79696-12-tzimmermann@suse.de