D3D11: Clean up buffer calls a bit. BUG=None Change-Id: I3b697c3b721b5be4888d039a5cefd160ab5b23f8 Reviewed-on: https://chromium-review.googlesource.com/367877 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>