workspaces-display: Improve pager autohide

The purpose of autohiding the workspace pager on the right was to
avoid exposure of workspaces to users who are not using them.
However, for users who do use workspaces, the behavior limits the
purpose of the overview. To fix, always show the pager if more than
one workspace is actively used.

https://bugzilla.gnome.org/show_bug.cgi?id=652714