Fix up and clarify use of maxMipLevel in Vulkan

Bug: skia:
Change-Id: I5b31e0c74be8f650e97c448b1029a2277dfee978
Reviewed-on: https://skia-review.googlesource.com/41844
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
5 files changed