Fix binding loop in KScreen KCM

We need to be careful when binding to transitive properties like enabled
or visible.

In this case our size changes to zero when the combobox visibility
changes this in turn changes our size which in turn changes the
formlayout to hide the layout item which changes the label visibility
which changes the layout item size...

Co-Authored-By: ivan tkachenko <me@ratijas.tk>
Co-Authored-By: David Edmundson <kde@davidedmundson.co.uk>