Vulkan: Change VK suffix to Vk 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>