monitor-manager: Don't try to match the outputs on hotplug

meta_monitor_config_match_current() only matches the number of outputs
and if the output connector, vendor, product and serial match.

This means that we can't use it to bypass doing any
work because it won't detect cases where we actually want to update
ourselves like e.g. an output being turned off either by us or by
another X client (e.g. xrandr).

https://bugzilla.gnome.org/show_bug.cgi?id=738630