Optimize mipmap downsample_2_2 in sRGB mode

Optimize mipmap downsample_2_2 in sRGB mode

Desktop (HP z620)
Before:
mipmap_build_2048x2048_0_gamma 10.5 ms
mipmap_build_2048x2048_1_gamma 77.1 ms
After:
mipmap_build_2048x2048_0_gamma 10.5 ms
mipmap_build_2048x2048_1_gamma 25.1 ms

Pixel XL
Before:
mipmap_build_2048x2048_0_gamma 160 ms
mipmap_build_2048x2048_1_gamma 1.5 s
After:
mipmap_build_2048x2048_0_gamma 160 ms
mipmap_build_2048x2048_1_gamma 313 ms

Also provides marginal performance improvements
for other sRGB downsamples.

BUG=skia:

Change-Id: Icfcd2ccd69676ccf3822db8042a4698e4464bb71
Reviewed-on: https://skia-review.googlesource.com/9386
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>