D3D11: minor cleanup to getSRVLevel(). Change getSRVLevel() to take an enum rather than a bool. Bug: angleproject:7303 Change-Id: I5b39b1b6e4f2f3f8a6bc8dd3f5690368f427f574 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3679482 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Stephen White <senorblanco@chromium.org>