Summary:
The currently minimum required Mesa version was 8.0 and KWin had special
caseing for Mesa 9.1. This is no longer realistic, no distribution
provides such an old Mesa version. So let's increase the version a
little bit.

Overview of Mesa versions supported by distributions:
 * Debian stable (stretch): 13.0
 * Debian oldstable (jessie): 10.3
 * Debian oldstable backports: 13.0
 * Debian oldoldstable (wheezy): 8.0
 * Ubuntu 16.04 LTS: 11.2
 * Ubuntu 16.04 LTS updates: 12.0
 * Ubuntu 17.04: 17.0
 * openSUSE Tumbleweed: 17.1
 * openSUSE Leap 42.2: 11.2
 * openSUSE Leap 42.3: 17.0
 * Arch: 17.1
 * Fedora 24: 12.0
 * Fedora 25: 17.0
 * Fedora 26: 17.1

Test Plan: Compiles

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6462