[graphite] Assume sources are already instantiated in copies

Change-Id: If265731bf1d6f94437321e5c99561a4de6210f76
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/845339
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
2 files changed