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>

drivers/gpu/drm/amd/amdgpu/amdgpu_object.h

@@ -87,8 +87,11 @@ struct amdgpu_bo {
 
 	struct ttm_bo_kmap_obj		dma_buf_vmap;
 	struct amdgpu_mn		*mn;
-	struct list_head		mn_list;
-	struct list_head		shadow_list;
+
+	union {
+		struct list_head	mn_list;
+		struct list_head	shadow_list;
+	};
 };
 
 /**