DisplayOzone doesn't really have anything related to Ozone.

Instead, it's the gbm platform that is used there. Thus,
rename it to DisplayGbm.

Bug: chromium:1084458
Change-Id: I4d974f6afbf0daa28fc3e83943b35814dca203fd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2211763Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>