Fix concentric 2pt conical gradient in GPU where r1 < r0

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Bug: skia:7683
Change-Id: I6608e72fa695c855af6a6a447885c4889b09fc1d
Reviewed-On: https://skia-review.googlesource.com/115042
Commit-Queue: Yuqian Li <liyuqian@google.com>
Reviewed-By: Greg Daniel <egdaniel@google.com>
Reviewed-on: https://skia-review.googlesource.com/117160
Reviewed-by: Yuqian Li <liyuqian@google.com>