Fix Vulkan Memory Allocator

See http://cl/931059450

Change-Id: I31cdb832d512051b2ef4e9232b97107932aa4db8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1266536
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>