Vulkan: Fix buffer size for vertex array * In syncDirtyEnabledAttrib() in VertexArrayVk, the array buffer size used for binding (BindVertexBuffers2()) has been changed to that of bufferHelper. Bug: angleproject:448047351 Change-Id: I852e4839cba698c01f92644cf32fadf366c0b54f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6996428 Commit-Queue: Cody Northrop <cnorthrop@google.com> Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>