drm/i915: fix possible refcount leak when resetting forcewake

If the timer putting the last forcewake refcount was pending and we
canceled it, we'll leak the corresponding forcewake and RPM references.

v2:
- do the ptr casting at the caller instead of adding a separate helper
  for this (Chris)
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_uncore.c

@@ -320,7 +320,8 @@ static void intel_uncore_forcewake_reset(struct drm_device *dev, bool restore)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	unsigned long irqflags;
 
-	del_timer_sync(&dev_priv->uncore.force_wake_timer);
+	if (del_timer_sync(&dev_priv->uncore.force_wake_timer))
+		gen6_force_wake_timer((unsigned long)dev_priv);
 
 	/* Hold uncore.lock across reset to prevent any register access
 	 * with forcewake not set correctly