egl-helpers: Support newer MESA versions

According to
https://www.khronos.org/registry/EGL/extensions/MESA/EGL_MESA_platform_gbm.txt
if MESA_platform_gbm is supported display should be initialized
from a GBM handle using eglGetPlatformDisplayEXT.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Message-id: 20170220095055.4234-1-fziglio@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
