v2:
  remove SUBPTB member
v3:
  remove last_level, use AMDGPU_VM_PTB directly instead.
Signed-off-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>