monitor-manager: Fix the max potential number of logical monitors

The max potential number of logical monitors (i.e. MetaMonitorInfos)
is the number of CRTCs, not the number of outputs.

In cases where we have more enabled CRTCs than connected outputs we
would end up appending more MetaMonitorInfos to the GArray than the
size it was initialized with which means the array would get
re-allocated rendering invalid some MetaCRTC->logical_monitor pointers
assigned previously and thus ending in crashes later on.

https://bugzilla.gnome.org/show_bug.cgi?id=751638