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