These dirty bits mostly track state needed for the command buffer. It
should speed up ContextVk::setupDraw by skipping unset dirty bits
instead of using boolean checks.

Improves perf slightly on the Vulkan null draw call test.

Bug: angleproject:2786
Change-Id: I071ce558c522732a17ad101d097ee4d10740abf1
Reviewed-on: https://chromium-review.googlesource.com/1194300
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>