Vulkan: Clean up ImageHelper::flushSingleSubresourceStagedUpdates This CL changed some logic to use helper function to make logic more clear. Bug: angleproject:42263375 Change-Id: I5d0ec0f6b0a315f9e755939420a655976a2fef5b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5620736 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Charlie Lao <cclao@google.com>