vulkan: Use host cached memory for read buffers

Investigation in https://crbug.com/432691010 found that memcpy() reading
data from a read only transfer buffer was ~10x slower with
Graphite/Dawn/Vulkan when compared to Ganesh/Vulkan. This was identified
as being caused by dawn using host coherent memory instead of host
cached.

Switch BufferVK to use host cached memory for read only transfer
buffers, aka those created with only BufferUsage::MapRead|Copydst, to
avoid this hangup. BufferVk also has to make explicit flush and
invalidate calls for non-host coherent memory.

Bug: 432691010
Change-Id: I723f19a75e67fff5b2e45e9bac20b9e948fd3014
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/254435
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Commit-Queue: Kyle Charbonneau <kylechar@google.com>