Vulkan: Use minimal dirty bits in VertexArrayVk. This should slightly reduce draw call overhead. BUG=angleproject:1898 Change-Id: I0e515bf2868f237f1d6948c12942f8cb6637c0c0 Reviewed-on: https://chromium-review.googlesource.com/707690 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>