For consistency with the rest of the backend.  Level index variables
were using the VK suffix while color index variables used Vk.

Bug: angleproject:4881
Change-Id: I0c2799da34cdfe19cb04adbebba042ac8876af96
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2413155
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>