monitor: Add scale 1 if no other supported scale was added

We currently have a hard coded limit on logical monitor sizes, meant
for filtering out monitor scales that would result in awkward desktop
sizes. This has the side effect of also disqualifying scale 1 for
resolutions that themself are lower than the mentioned limit. To avoid
listing no supported scales, always add the fallback scale 1 if no
other was added.

https://bugzilla.gnome.org/show_bug.cgi?id=787477