Finish up mip opts: 2x3 and 3x2

Finish up mip opts: 2x3 and 3x2

mipmap_build_2048x2047 (2x3):
sRGB Float Impl (old): 82.9ms (reference)
8888                 : 12.3ms (reference)
sRGB Before          : 61.9ms
sRGB After           : 53.1ms

mipmap_build_2047x2048 (3x2):
sRGB Float Impl (old): 65.9ms (reference)
8888 Before          : 10.3ms
8888 After           : 8.81ms
sRGB Before          : 47.8ms
sRGB After           : 43.5ms

BUG=skia:

Change-Id: I53ef01e8b8344f018aa940d6c80cf2cf048bf7fa
Reviewed-on: https://skia-review.googlesource.com/10028
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>