drm/amdgpu: Keep a bo-reference per-attachment

For now they all reference the same BO. For correct DMA mappings they will
refer to different BOs per-GPU.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Ramesh Errabolu <Ramesh.Errabolu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>