The only difference between the NONRENDERABLE and normal versions of
the formats is the rtvFormat is blank for non-renderable. The only
place that I can see we check rtvFormat to make a decision is in
TextureStorage11::GetTextureBindFlags. The rtvFormat here isn't
necessary because we also check the 'renderTarget' flag, which won't
be true for compressed formats at any time.

BUG=angleproject:1455

Change-Id: Icbb438d336e67b80c44a0ab8de995a32b92e451e
Reviewed-on: https://chromium-review.googlesource.com/359085Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>