On AMD systems the driver was reporting these textures as usable.
This was causing crashes in the sRGB extension tests.

BUG=angle:839

Change-Id: I8e9924ed44f82a2a6bc891d1df76c0f9a19084ad
Reviewed-on: https://chromium-review.googlesource.com/232062Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>