As we are approaching the point when DisplayGLX will be compiled
in Ozone/X11 configuration, we need to resolve compilation errors.

Ozone has intptr_t EGLNativeDisplayType whereas X11 requires
Display* aka _XDisplay*. We've already done so in Chromium, and
it's time to do the same cast in angle.

Bug: chromium:1084458
Change-Id: I11b5ae2e10bfa316cbaa882be86af6894cfdbbf2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2211761Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>