D3D11: Fix uninitialized BufferEx SRV. This was causing an unrelated error that was complicating diagnosing test failures with the command graph refactor. Bug: angleproject:4029 Change-Id: I31df76e7ea1c7fb731b49e97ac8a527a165efd48 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2024331 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>