This method returned a sampler state as normal, with the base level
automatically offset by the one or two levels we use in the D3D
workaround for small compressed textures. Since this is D3D-only,
we can move all of the logic into the D3D classes themselves.

BUG=angle:781

Change-Id: Ie0a60877efebb41f02f4e57625a44e5fb5cce074
Reviewed-on: https://chromium-review.googlesource.com/222923Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>