Revert "Move DstCopy on gpu into the GrXferProcessor." This reverts commit 74a11753604768bf461b80cabb66060e8564d82c. TBR=joshualitt@google.com,bsalomon@google.com NOPRESUBMIT=true NOTREECHECKS=true NOTRY=true BUG=skia: Review URL: https://codereview.chromium.org/896163003