monitor: Handle tiled monitors where (0, 0) is not the main output

In some circumstances, the origin tile (0, 0) is not the one that
should be used to drive the monitor when using a non-tiled mode. Update
MetaMonitorTiled to support this case. It also seems to be so that the
preferred mode might be some low resolution or bogus mode on these
monitors, so also adapt MetaMonitorTiled to manage to ignore the
preferred mode of a tiled monitor if the preferred mode doesn't use
both tiles.

https://bugzilla.gnome.org/show_bug.cgi?id=781723