Fix allowing CompressedTexSubImage calls on ETC1 texture types
(disallowed in the extension spec).

BUG=angleproject:1523

Change-Id: Ic90175ff4626da0170b6c94f204a9d31fd0154a7
Reviewed-on: https://chromium-review.googlesource.com/744443Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>