drm/probe: Fix drm_connector_helper_hpd_irq_event deadlock

drm_connector_helper_hpd_irq_event() calls
drm_kms_helper_hotplug_event() with the mode-setting lock taken while
it's supposed to be called without that lock taken.

This results in a lockdep warning, and a deadlock if we were to wake up
a TV through CEC (and possibly other cases).

Signed-off-by: Maxime Ripard <maxime@cerno.tech>