drm/i915: Only WARN about a stuck hotplug irq ONCE

It seems that hardware that is broken enough to emit a hotplug IRQ even
though the pin is surposedly disable, will do so indefinitely.

Note: There's a good chance the underlying issue has been fixed with

commit 0ce99f74
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jul 26 11:27:49 2013 +0200

    drm/i915: fix gen4 digital port hotplug definitions

References: https://bugzilla.redhat.com/show_bug.cgi?id=1051170
Link: https://bugzilla.redhat.com/attachment.cgi?id=847786Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
[danvet: Add note about the potential fix.]
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -1233,8 +1233,8 @@ static inline void intel_hpd_irq_handler(struct drm_device *dev,
 	spin_lock(&dev_priv->irq_lock);
 	for (i = 1; i < HPD_NUM_PINS; i++) {
 
-		WARN(((hpd[i] & hotplug_trigger) &&
-		      dev_priv->hpd_stats[i].hpd_mark != HPD_ENABLED),
+		WARN_ONCE(hpd[i] & hotplug_trigger &&
+			  dev_priv->hpd_stats[i].hpd_mark != HPD_ENABLED,
 			  "Received HPD interrupt although disabled\n");
 
 		if (!(hpd[i] & hotplug_trigger) ||