Minor cleanups to GrMtlGpuRTCommandBuffer

Bug: skia:8243
Change-Id: I80ed04f7bfbcf20549de30822245d8fe2832a810
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/219996
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed