D3D11: Alias dirty buffer channel bindings. The types for this were a bit disorganized. Clean this up. BUG=angleproject:1635 Change-Id: I669d347697e0c7c5b06ed7165fa0997c375e876e Reviewed-on: https://chromium-review.googlesource.com/483315 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>