drm/i915: Fixup for 'Hold mode_config->mutex during hotplug'

drm_helper_hpd_irq_event queues another work proc to go and deliver
the user-space event, and that function also wants to hold the config
mutex, so we shouldn't hold the mutex across the
drm_helper_hpd_irq_event call.

Signed-off-by: Keith Packard <keithp@keithp.com>