Never use CPU-built mips for Gray_8.

Fixes a bug where Gray_8 textures would still render as Alpha_8 (if the
first draw was detected as needing mips).

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2088883002

Review-Url: https://codereview.chromium.org/2088883002
1 file changed