Don't use GENERAL layout during mipmap generation in Vulkan

With the latest validation layers, they now spew an annoying number of
warnings cause we are using GENERAL when making mip maps. Even though
there was technically nothing wrong with our current code, this updates
the generation to use the specific layouts instead.

Bug: skia:
Change-Id: I0ce431e2540fd6fb2a87134768ca3ed488661012
Reviewed-on: https://skia-review.googlesource.com/112061
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed