[graphite] Simplify prep code around calling UploadSource::Make Bug: b/333909822 Change-Id: I2e44c56364840c092c1b8a6885ff6110f4a4e0fc Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1222817 Reviewed-by: Robert Phillips <robertphillips@google.com> Reviewed-by: Nicolette Prevost <nicolettep@google.com> Commit-Queue: Michael Ludwig <michaelludwig@google.com>