D3D11: Fix loading of RGBA4 textures for ubyte. BUG=angleproject:1407 BUG=chromium:616176 Change-Id: Ibb7c5e71b4cf8afe7b7d0092e7d019b87d507ba6 Reviewed-on: https://chromium-review.googlesource.com/350905 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>