[graphite] Revise Graphite Perlin noise shader code

Bug: b/238754013
Change-Id: I14b297480cbd56d757cb695d4d93ee60c3734f6c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/654036
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed