d3d11::TextureFormat and d3d11::FormatSet don't really do different
things, so it would be best to merge them.

BUG=angleproject:1455

Change-Id: I8c09ae763e4cb284bb611fed9d74b9048fb9f13e
Reviewed-on: https://chromium-review.googlesource.com/367091Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>