In later CLs FunctionsGLX will need to store the X display so we take
advantage of it to reduce the verbosity of the other GLX files slightly.

BUG=angleproject:892

Change-Id: I42ea00d0a37055e5e0752a860978b8ef5afb7a0b
Reviewed-on: https://chromium-review.googlesource.com/271163Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>