SkSplicer: fix dstover

dstover was updating the {dr,dg,db,da} registers instead of {r,g,b,a}.

Change-Id: Ie2280abe80efb274fb422dc38a329c6dac12f846
Reviewed-on: https://skia-review.googlesource.com/8404
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed