Vulkan: Optimize ContextVk::setupDraw. This improves performance significantly in the Vulkan CPU overhead test. Bug: angleproject:2786 Change-Id: I911bc66a6b2d11dd3848ffa90927b314aeadfc24 Reviewed-on: https://chromium-review.googlesource.com/1194301 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>