monitor-manager: Avoid crashing on GPUs with no CRTCs

Fixes: https://bugs.launchpad.net/bugs/1822616

Because the GPU has no CRTCs (which is an Nvidia bug for nvidia-drm.modeset=1):

$ xrandr --verbose
Screen 0: minimum 8 x 8, current 640 x 480, maximum 16384 x 16384