Add the missing shift to the dy

This is identical to https://skia-review.googlesource.com/c/5266/ except for
the SK_ANALYTIC_AA_GUARD flag.

BUG=chromium:668907

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=5302

Change-Id: I3fc225a925d21fe615c46a4a0be7fe33c5790766
Reviewed-on: https://skia-review.googlesource.com/5302
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>