Vulkan: Enable persistently mapped buffer objects The VMA allocator has a handy feature where during memory allocation we can request persistently mapped memory. This saves IOCTL overhead for apps that update buffers frequently. Bug: angleproject:2162 Change-Id: I870d880033beec343efae6de06f1c5935de4c2c1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2155131 Reviewed-by: Tobin Ehlis <tobine@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Tobin Ehlis <tobine@google.com>