drm/mgag200: vga-bmc: Control CRTC VIDRST flag from encoder

Control the VIDRST pin from the VGA-BMC encoder's atomic_check and
remove the respective code from CRTC. Makes the VIDRST functionality
fully composable.

The VIDRST pin allows an external clock source to control the SYNC
signals of the Matrox chip. The functionality is part of the CRTC,
but depends on the presence of the clock source. This is the case for
some BMCs, so control the pin from the VGA-BMC output.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240805130622.63458-4-tzimmermann@suse.de