Steam Machine CEC: find amdgpu's HDMI port with a single CEC port.

amdgpu registers its HDMI notifier without a port name, which only matches
the driver's named lookup ("Port C") if the CEC driver loaded first; amdgpu
loads from the initramfs, so it never does and /dev/cec0 stays at f.f.f.f.
With a single CEC port, look the notifier up by device alone.

Applies to Valve's drivers/media/cec/platform/cros-ec/cros-ec-cec.c
(evlaV/linux-integration 10c8c880, see CEC_DRIVER_URL in lib/cec.sh).

--- a/cros-ec-cec.c
+++ b/cros-ec-cec.c
@@ -485,7 +485,7 @@
 		goto out_probe_adapter;
 	}
 
-	port->notify = cec_notifier_cec_adap_register(hdmi_dev, conns[port_num],
+	port->notify = cec_notifier_cec_adap_register(hdmi_dev, conns[1] ? conns[port_num] : NULL,
 						      port->adap);
 	if (!port->notify) {
 		ret = -ENOMEM;