monitor-manager-xrandr: Disable CRTCs if there is no monitor

After last monitor gets unplugged from the system, hotplug detection may
no longer work on Intel GFX.

This is because we didn't trigger a modeset to disable CRTCs, and i915
requires it to make hotplug detection continue to work [1].

There's no guarantee that DPMS off in DDX also disables CRTCs, so
explicitly disable CRTCs to solve the issue.

[1] https://www.kernel.org/doc/html/latest/gpu/i915.html#hotplug

https://gitlab.freedesktop.org/drm/intel/-/issues/2602

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1561>