drm/amdkfd: Add more comments on GFX9 user CP queue MQD workaround

Because too many things are involved in this workaround, we need more
comments to avoid pitfalls.

Signed-off-by: Yong Zhao <Yong.Zhao@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Philip Yang <philip.yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>