kcm_opengl: Fix EGL info retrieval.

Summary:
Under certain contexts (tested with Mesa 18.0.1 + AMDGPU) eglMakeCurrent()
fails with EGL_BAD_ACCESS. At this point the KCM gives up on EGL info
retrieval. This seems to be caused - correctly or otherwise - by a dangling
glX context. Explictly setting no glX context current at cleanup fixes the
issue.

Test Plan: kcmshell5 opengl should correctly display EGL information.

Reviewers: #plasma, davidedmundson, graesslin

Reviewed By: #plasma, davidedmundson, graesslin

Subscribers: plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12455