D3D: minor cleanup for image load/store. Change-Id: I633d51f01845b92f864df1229d501d572b96b314 Bug: angleproject:7121 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3638991 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Stephen White <senorblanco@chromium.org>