Instead of the double linked list. Gets the size of amdgpu_vm_pt down to
64 bytes again.

We could even reduce it down to 32 bytes, but that would require some
rather extreme hacks.
Signed-off-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NChunming Zhou <david1.zhou@amd.com>
Acked-by: NFelix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>