monitor: Use current monitor mode to check whether active
For historical reasons meta_monitor_is_active() checked whether it is active by checking whether the main output have a CRTC assigned and whether that CRTC has a current mode. At a later point, the MetaMonitor got its own mode abstraction (MetaMonitorMode), but meta_monitor_is_active() was never updated to use this.
An issue with checking the main output's CRTC state is that, if there is some CRTC mode combination that for some reason isn't properly detected by the MetaMonitorMode abstraction (e.g. some tiling configuration not yet handled), meta_monitor_is_active() would return TRUE, even though no (abstracted) mode was set. This would cause confusion here and there, leading to NULL pointer dereferences due to the assumption that if a monitor is active, it has an active mode.
Instead, change meta_monitor_is_active() to directly check the current monitor mode, and log a warning if the main output still happen to have a CRTC with a mode assigned to it. This way, when an not undrestood CRTC mode combination is encountered, instead of dereferencing NULL pointers, simply assume the monitor is not active, which means that it will not be managed or rendered by mutter at all.