drm/amdgpu: fix amdgpu_vm_fini

We should not remove mappings in rbtree_postorder_for_each_entry_safe
because that rebalances the tree.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>