hide primary display combo when not supported
Some backends, for example the kwayland backend, do not support
setting the primary display. The backends already make their
feature set known so we can just hide it from the UI.
It's enough to hide it from the UI since setting it doesn't make
any difference, the libkscreen backend will simply ignore it
internally, so it's just honest to not show it.
ran kscreen kcm on both, X11 and wayland platforms, combo
and label are shown and hidden as expected
Reviewers: Plasma, dvratil, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: kpiwowarski, plasma-devel
Differential Revision: https://phabricator.kde.org/D10723