The Vulkan backend creates an image with 3 mips, while it should only
create one with a single mip.

Bug: angleproject:4686
Change-Id: Ide7f9612c9440bfb9d9650637f6a1ad42be61208
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2230797Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>