gpu-compute: Init CU object for pipe stages in their ctors

gpu-compute: Init CU object for pipe stages in their ctors

This change updates the constructors of the CU's pipe
stages/memory pipelines to accept a pointer to their
parent CU. Because the CU creates these objects, and
can pass a pointer to itself to these object via their
constructors, this is the safer way to initalize these
classes.

Change-Id: I0b3732ce7c03781ee15332dac7a21c097ad387a4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29945
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>