Since 9.3 doesn't support non-zero level render targets, we should
always be getting the level zero render target. Previously the code
would never try creating a render target when level zero mode was
not enabled, but this can break if we refactor the code.

Fixes a bug in a future patch which creates the storage as a render
target instead of as a non-renderable texture first.

BUG=angleproject:2107

Change-Id: Ib2732a35e586bc4956d07783325429e8edeb0c66
Reviewed-on: https://chromium-review.googlesource.com/614120Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>