workspaces-view: Don't create extra workspaces unconditionally

Extra workspaces are special, in that they collect windows from
all workspaces for a particular monitor. This matches the default
behavior, but we need more than a single workspace per monitor if
workspaces-only-on-primary is false, so don't create the extra
workspaces in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=652580