Clean up logic in fontregen GM

Since this is a GpuGM we're guaranteed to have a GrContext

Change-Id: Ibab90a8734ebf36ed2bcae534e67e13894a18420
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/197766
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>