Vulkan: Clean up shader buffer DS allocation. We were using repeated code patterns in several places. Bug: angleproject:5736 Change-Id: I206783dd792c3b0f9d38027235445ec600a60d88 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2773320 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>