arch-gcn3,gpu-compute: Update stats style for GPU

arch-gcn3,gpu-compute: Update stats style for GPU

Convert all gpu-compute stats to Stats::Group style.

Change-Id: I29116f1de53ae379210c6cfb5bed3fc74f50cca5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39135
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matthew Poremba <matthew.poremba@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>