drm/amdgpu: move shadow_list to amdgpu_bo_vm

Move shadow_list to struct amdgpu_bo_vm as shadow BOs
are part of PT/PD BOs.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>