Some SkPM4f cleanup

Bug: skia:
Change-Id: I6a06b2ff951b9a1319c40b16b33840e220fecc37
Reviewed-on: https://skia-review.googlesource.com/155900
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
12 files changed