drm/i915: Synchronize hpd work in i915_hpd_storm_ctl_show()

While trying to add a chamelium test for short HPD IRQs, I ran into
issues where a hotplug storm would be triggered, but the point at which
it would be reported by the kernel would be after igt actually finished
checking i915_hpd_storm_ctl's status. So, fix this by simply
synchronizing our IRQ work, dig_port_work, and hotplug_work before
printing out the HPD storm status in i915_hpd_storm_ctl_show().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121003718.17704-1-lyude@redhat.com