D3D11: Disable the RGB10A2 backbuffer format. Chrome was using this format by accident for video decode surfaces, resulting in a power usage regression. BUG=angleproject:1662 BUG=691106 Change-Id: I3b9ac814f06c07490fceb88e8b087b235e5b25bf Reviewed-on: https://chromium-review.googlesource.com/441950 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>