Follow the preferred mode if we're asked to (i.e. if under QXL)
Summary:
As described in D15789, follow the preferred mode when the backend
asks us to.
Test Plan: tested together with the libkscreen patch
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D15790