[platforms/x11] Request OpenGL 2.1 instead of 1.2

Summary:
Mismatch in the major/minor version. Requesting 1.2 doesn't make any
sense given that KWin requires 2.1.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D6401