Vulkan: Move blit mipmap init to ImageHelper. This hides the logic for manipulating the current layout and barriers into the image helper. This will make it easier to implement implicit pipeline barriers. It allows us to remove the updateLayout() hack. Bug: angleproject:2828 Change-Id: I3e59872e66064e6b105c1f398b4212fb3a6be1a7 Reviewed-on: https://chromium-review.googlesource.com/1255506 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>