drm/vc4: Fall back to using an EDID probe in the absence of a GPIO.

On Pi0/1/2, we use an external GPIO line for hotplug detection, since
the HDMI_HOTPLUG register isn't connected to anything.  However, with
the Pi3 the HPD GPIO line has moved off to a GPIO expander that will
be tricky to get to (the firmware is constantly polling the expander
using i2c0, so we'll need to coordinate with it).

As a stop-gap, if we don't have a GPIO line, use an EDID probe to
detect connection.  Fixes HDMI display on the pi3.
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -174,6 +174,9 @@ vc4_hdmi_connector_detect(struct drm_connector *connector, bool force)
 			return connector_status_disconnected;
 	}
 
+	if (drm_probe_ddc(vc4->hdmi->ddc))
+		return connector_status_connected;
+
 	if (HDMI_READ(VC4_HDMI_HOTPLUG) & VC4_HDMI_HOTPLUG_CONNECTED)
 		return connector_status_connected;
 	else