drm/mgag200: vga-bmc: Control BMC scanout from encoder

Move calls to stop and start BMC scanout from CRTC helpers to the
VGA-BMC encoder's atomic_disable and atomic_enable. Makes the BMC
scanout transparent to the CRTC.

DRM's atomic helpers call an encoder's atomic_disable and atomic_enable
helpers for all enabled encoders. The BMC stops scanning out the VGA
signal if modeset disables the VGA encoder, and starts scanning out
if the modeset enables the VGA encoder.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240805130622.63458-5-tzimmermann@suse.de