On HLSL 4+, the debug runtime issues a warning when we try to render
to a color output in the pixel shader that doesn't have a matching
render target bound. This happens when doing a depth or stencil-only
render pass. We only need to bind a dummy output in HLSL 3, so tighten
the workaround we had in place and fix the warning for D3D11.

BUG=angleproject:2025

Change-Id: I16ba9e907f3a6e59afff93fe4583d084cbdf42c5
Reviewed-on: https://chromium-review.googlesource.com/617268Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>