[graphite] Make GM:gpuSetup() more backend agnostic.

Bug: b/238756380
Change-Id: I479798fbf26ccb384b8ccef7c77f38f2c7e3def4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/642736
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
22 files changed