Metal backend is not ready yet, so report as unavailable
to avoid DisplayMtl being created by frontend.

Bug: angleproject:2634
Change-Id: Ia45977cede01f62347967eefe661b63bef3575e5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1865736
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>