drm/i915: Only call mod_timer() if not already pending

The final arrangement of updating timer->expires and calling mod_timer()
used in

commit 672e7b7c
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Nov 19 09:47:19 2014 +0000

    drm/i915: Don't continually defer the hangcheck

turns out to be very unsafe. Try again.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -3067,9 +3067,10 @@ void i915_queue_hangcheck(struct drm_device *dev)
 		return;
 
 	/* Don't continually defer the hangcheck, but make sure it is active */
-	if (!timer_pending(timer))
-		timer->expires = round_jiffies_up(jiffies + DRM_I915_HANGCHECK_JIFFIES);
-	mod_timer(timer, timer->expires);
+	if (timer_pending(timer))
+		return;
+	mod_timer(timer,
+		  round_jiffies_up(jiffies + DRM_I915_HANGCHECK_JIFFIES));
 }
 
 static void ibx_irq_reset(struct drm_device *dev)