Vulkan: Use copyImageDataToBuffer in Texture mipmap gen. There were two code paths using nearly identical code. Fix this by generalizing the helper method slightly. Refactoring change only. Bug: angleproject:2828 Change-Id: I39caccfaad4880e566e995483b8e8fb46d67d3da Reviewed-on: https://chromium-review.googlesource.com/1249563 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>