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

drivers/gpu/drm/i915/i915_debugfs.c

@@ -4592,6 +4592,13 @@ static int i915_hpd_storm_ctl_show(struct seq_file *m, void *data)
 	struct drm_i915_private *dev_priv = m->private;
 	struct i915_hotplug *hotplug = &dev_priv->hotplug;
 
+	/* Synchronize with everything first in case there's been an HPD
+	 * storm, but we haven't finished handling it in the kernel yet
+	 */
+	synchronize_irq(dev_priv->drm.irq);
+	flush_work(&dev_priv->hotplug.dig_port_work);
+	flush_work(&dev_priv->hotplug.hotplug_work);
+
 	seq_printf(m, "Threshold: %d\n", hotplug->hpd_storm_threshold);
 	seq_printf(m, "Detected: %s\n",
 		   yesno(delayed_work_pending(&hotplug->reenable_work)));