Add half-float mipmap benchmarks too

Add half-float mipmap benchmarks too

Just trying to get better perf coverage of our mipmapping code, in
anticipation of different optimization strategies for the sRGB case.
I want to have some good reference points to measure progress.

BUG=skia:

Change-Id: I62e2d025fdcf9f12e6c86658c6ce7320f14fcc6f
Reviewed-on: https://skia-review.googlesource.com/9323
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>