drm/amdgpu: shadow and mn list are mutually exclusive

Save some memory because only one of those is used at all times.

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