Invoke the callback only when DeviceInfo is available.

- There is possibility that DeviceInfo can be null when unplugging MHL cable,
  if the capability register event is missed. and we should not invoke
  the callback in this case.
- In addition, we'd better use the portId of HdmiMhlLocalDevice directly,
  not one of DeviceInfo.

Bug: 17366541
Change-Id: I40d71e27d82c5f3a4c7aabda32119856c0bde83d