gpu-compute: Add pipeline stage interface classes

gpu-compute: Add pipeline stage interface classes

This change separates the pipeline stage interfaces
for the GPU's compute unit into their own classes
with a well-defined interface. This helps to create
a cleaner interface for users to extend the CU
pipeline's capabilities and also helps consolidate
all the pipeline communication code in one place
in the source.

Change-Id: I569d52bce84dc1b9fbf8f0f96d53a81a2b6773c6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29972
Reviewed-by: Anthony Gutierrez <anthony.gutierrez@amd.com>
Maintainer: Anthony Gutierrez <anthony.gutierrez@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>