display: Use virtual clone modes when mirroring

These are not real modes, but just as place holders when generating
'mirror' configurations. The clone modes will be just to match the
flag/dimension, while the actual mode applied will be individual for
each monitor.

This allows monitors to have their own refresh rates, which is possible
since a few mutter versions back. This also matches how mutter itself
generates mirror modes when doing so via the key binding.