Properly detect Gallium drivers with newer Mesa

Summary:
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

Tags: KWin

Differential Revision: https://phabricator.kde.org/D9495