output-management: add a way to trigger a rescan

According to a recent discussion on dri-devel, userspace should provide a way to force a full reprobe of the DRM connectors:

User opens the screen configuration tool -> usually at that point the tool/compositor force a full reprobe, which then often triggers the automatic reconfiguring. E.g. on one laptop I have here when I plug in random shit projectors at conferences nothing happens, until I run xrandr, which triggers the full reprobe, which then makes the kernel realize something change, sending and uevent, which starts the automatic reconfigure machinery.

This is really not great, but the kernel folks say there's no way around that.