drm/mgag200: Only set VIDRST bits in CRTC modesetting

The VRSTEN and HRSTEN bits control whether a CRTC synchronizes its
display signal with an external source on the VIDRST pin. The G200WB
and G200EW3 models synchronize with a BMC chip, but different external
video encoders, such as the Matrox Maven, can also be attached to the
pin.

Only set VRSTEN and HRSTEN bits in the CRTC mode-setting code, so the
bits are independent from the BMC. Add the field set_vidrst to the CRTC
state for this purpose. Off by default, control the CRTC VIDRST setting
from the CRTC's atomic_check helper.

v3:
- don't clear bits unnecessary (Jocelyn)
v2:
- keep logic entirely in CRTC (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240711072415.11831-2-tzimmermann@suse.de