Also fixes the check for the correct UINT index extension that wasn't
available on Vulkan. Also includes a workaround for the mock ICD not
implementing buffer state for index ranges.

Bug: angleproject:2923
Change-Id: Iab35809d15f890525a9e658d4148272c46cf1320
Reviewed-on: https://chromium-review.googlesource.com/c/1308733Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>