[graphite] Centralize Graphite onMakeTextureImage stubs in SkImage_GpuBase

This is just some minor refactoring/cleanup from an omnibus CL.

Bug: b/237111152
Change-Id: I09e6ab2c975b6c45eda4258cba728b237e398dec
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/627697
Reviewed-by: James Godfrey-Kittle <jamesgk@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
6 files changed