Redraw the slider if modes have changed
Summary:
This can happen if a monitor was not configured at all, the KCM is shown
when it first plugged in and the user selects to not configure it.
In the UI it will appear with no Resolution/Refresh rate and wrong size in
the QML view.
Backport of D17658
Depends on D17685
Reviewers: Plasma, dvratil
Reviewed By: dvratil
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D17686