drm/radeon: Use ttm_bo_move_null() in radeon_bo_move()

Since ttm_bo_move_null() is exactly the same as ttm_resource_free() +
ttm_bo_assign_mem(), we use ttm_bo_move_null() for the GTT --> SYSTEM
move case too. Then the code is more consistent as the SYSTEM --> GTT
move case.

Acked-by: Christian König <[e-mail]>
Signed-off-by: Huacai Chen <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/radeon_ttm.c

···
 	if (old_mem->mem_type == TTM_PL_TT &&
 	    new_mem->mem_type == TTM_PL_SYSTEM) {
 		radeon_ttm_tt_unbind(bo->bdev, bo->ttm);
-		ttm_resource_free(bo, &bo->resource);
-		ttm_bo_assign_mem(bo, new_mem);
+		ttm_bo_move_null(bo, new_mem);
 		goto out;
 	}
 	if (rdev->ring[radeon_copy_ring_index(rdev)].ready &&