Properly detect Gallium drivers with newer Mesa
The renderer string does not contain "Gallium 0.4 on" anymore,
instead it directly contains the gallium driver's name.
So assume that every unknown renderer is a gallium driver.
Test Plan: Added a testcase, it succeeds only with this patch.
Reviewers: Plasma, graesslin
Subscribers: kwin, plasma-devel, KWin
Differential Revision: https://phabricator.kde.org/D9495