drm/vc4: hdmi: Only call into DRM framework if registered

Our hotplug handler will currently call the drm_kms_helper_hotplug_event
every time a hotplug interrupt is called.

However, since the device is registered after all the drivers have
finished their bind callback, we have a window between when we install
our interrupt handler and when drm_dev_register() is eventually called
where our handler can run and call drm_kms_helper_hotplug_event but the
device hasn't been registered yet, causing a null pointer dereference.

Fix this by making sure we only call drm_kms_helper_hotplug_event if our
device has been properly registered.

Fixes: f4790083 ("drm/vc4: hdmi: Rely on interrupts to handle hotplug")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210707095112.1469670-4-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -1580,7 +1580,7 @@ static irqreturn_t vc4_hdmi_hpd_irq_thread(int irq, void *priv)
 	struct vc4_hdmi *vc4_hdmi = priv;
 	struct drm_device *dev = vc4_hdmi->connector.dev;
 
-	if (dev)
+	if (dev && dev->registered)
 		drm_kms_helper_hotplug_event(dev);
 
 	return IRQ_HANDLED;