drm/mgag200: Fix value in <VBLKSTR> register

Fix an off-by-one error when setting the vblanking start in
<VBLKSTR>. Commit d6460bd5 ("drm/mgag200: Add dedicated
variables for blanking fields") switched the value from
crtc_vdisplay to crtc_vblank_start, which DRM helpers copy
from the former. The commit missed to subtract one though.

Reported-by: Wakko Warner <wakko@animx.eu.org>
Closes: https://lore.kernel.org/dri-devel/CAMwc25rKPKooaSp85zDq2eh-9q4UPZD=RqSDBRp1fAagDnmRmA@mail.gmail.com/


Reported-by: Сергей <afmerlord@gmail.com>
Closes: https://lore.kernel.org/all/5b193b75-40b1-4342-a16a-ae9fc62f245a@gmail.com/
Closes: https://bbs.archlinux.org/viewtopic.php?id=303819


Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: d6460bd5 ("drm/mgag200: Add dedicated variables for blanking fields")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.12+
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Wakko Warner <wakko@animx.eu.org>
Link: https://lore.kernel.org/r/20250416083847.51764-1-tzimmermann@suse.de