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/nouveau/nv50_instmem.c b/drivers/gpu/drm/nouveau/nv50_instmem.c
index f5800f2..a53fc97 100644
--- a/drivers/gpu/drm/nouveau/nv50_instmem.c
+++ b/drivers/gpu/drm/nouveau/nv50_instmem.c
@@ -347,7 +347,7 @@
 		return ret;
 	}
 
-	gpuobj->vinst = gpuobj->im_backing->bo.mem.mm_node->start << PAGE_SHIFT;
+	gpuobj->vinst = gpuobj->im_backing->bo.mem.start << PAGE_SHIFT;
 	return 0;
 }