drm/ttm: restructure to allow driver to plug in alternate memory manager

Nouveau will need this on GeForce 8 and up to account for the GPU
reordering physical VRAM for some memory types.

Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Acked-by: Thomas Hellström <thellstrom@vmware.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/ttm/ttm_bo_util.c b/drivers/gpu/drm/ttm/ttm_bo_util.c
index 0ebfe0d..c9d2d4d 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_util.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_util.c
@@ -256,8 +256,7 @@
 	dir = 1;
 
 	if ((old_mem->mem_type == new_mem->mem_type) &&
-	    (new_mem->mm_node->start <
-	     old_mem->mm_node->start + old_mem->mm_node->size)) {
+	    (new_mem->start < old_mem->start + old_mem->size)) {
 		dir = -1;
 		add = new_mem->num_pages - 1;
 	}