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