remove loop in powf

Looking at the diff, I don't think we'd land the CL in the other direction.

Bug: chromium:842374

Change-Id: I9cbfbcbabbe6a3b36c1369e1ab81ff953cfb3c0b
Reviewed-on: https://skia-review.googlesource.com/128004
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@chromium.org>
37 files changed