drm backend: choose correct EGL config with mesa-18

Summary:
Do not blindly select first EGL config from returned list, but choose the one that matches GBM surfaces, that will be created later.
GBM surfaces are created with GBM_FORMAT_XRGB8888 format, so choose the config that matches it.
With wrong format EglGbmBackend::resetOutput() will later fail with error EGL_BAD_MATCH.

Test Plan: Compile, run startplasmacompositor. Verify that OpenGL compositing is used, either by kwin debug console, or by kwin support information.

Reviewers: graesslin, davidedmundson, #kwin, #plasma_on_wayland, bshah

Reviewed By: davidedmundson

Subscribers: zzag, kwin, #kwin

Tags: #kwin, #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D11758