Never use CPU-built mips for Gray_8.
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

Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,aosp-mirror/platform_external_skia,rubenvb/skia,rubenvb/skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia