simplify GrAtlasTextOp's onPrepare

Change-Id: Ib9fa2d9300a5bc7bfe7231cd858128a79a93bb48
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/302839
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
1 file changed