check for EGL_KHR_platform_gbm extension as well

Summary:
some drivers, like Mali have EGL_KHR_platform_gbm
but not EGL_MESA_platform_gbm

Test Plan: pending a test on rock64 board

Reviewers: KWin, Plasma, davidedmundson, graesslin

Reviewed By: KWin, Plasma, graesslin

Subscribers: graesslin, garg, davidedmundson, plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D10346