Re-enable CPU mipmap generation for Ganesh. Aniso mips were landed a while ago. Howev...

Re-enable CPU mipmap generation for Ganesh. Aniso mips were landed a while ago. However, the CPU builder fails when it sees Index8 (among other things), so change the code to fallback to GPU in that case. Additionally, if we're going to be mipping an sRGB image, don't use the CPU code (which is not yet gamma correct). Unfortunately, this means that we will often be using the GPU path, still - with recent codec changes, most images are coming in tagged as sRGB.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1835003002

Review URL: https://codereview.chromium.org/1835003002