EGL: Add pbuffer support for drm platform
Add pbuffer support for drm platform, because some customers are still using this feature.
V2: - Rebase https://patchwork.freedesktop.org/patch/51719/ resp.
https://lists.freedesktop.org/archives/mesa-dev/2015-June/086358.html
- Drop ancient RB
- This comment from Chad still needs addressing
https://lists.freedesktop.org/archives/mesa-dev/2015-June/086741.html