remove SK_API from private calls

It's usually better to use __fp16.

Change-Id: I9db8983cb47cbd5bfe99ae76bf8e4b42bb7240a2
Reviewed-on: https://skia-review.googlesource.com/c/185980
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
1 file changed