gpu-compute: Use refs to CU in pipe stages/mem pipes

gpu-compute: Use refs to CU in pipe stages/mem pipes

The pipe stages and memory pipes are changed to store
a reference to their parent CU as opposed to a pointer.
These objects will never change which CU they belong to,
and they are constructed by their parent CU.

Change-Id: Ie5476e1e2e124a024c2efebceb28cb3a9baa78c1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29969
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>