drm/i915: fix possible RPM ref leaking during RPS disabling

In

commit c6df39b5ea6342323a42edfbeeca0a28c643d7ae
Author: Imre Deak <imre.deak@intel.com>
Date:   Mon Apr 14 20:24:29 2014 +0300

    drm/i915: get a runtime PM ref for the deferred GT powersave enabling

I added an RPM get-ref when enabling RPS from a deferred work, but forgot
to add the corresponding put-ref when canceling the work. This may leave
RPM disabled.

Note that the race is real since we run the rps enabling with a
delayed work item after resume, so leaves enough time (in contrived
examples) to fit a quick autoresum in.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Robert Beckett <robert.beckett@intel.com>
Testecase: igt/pm_rpm/system-suspend
[danvet: Mention testcase and add note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4646,7 +4646,9 @@ void intel_disable_gt_powersave(struct drm_device *dev)
 		ironlake_disable_drps(dev);
 		ironlake_disable_rc6(dev);
 	} else if (IS_GEN6(dev) || IS_GEN7(dev) || IS_BROADWELL(dev)) {
-		cancel_delayed_work_sync(&dev_priv->rps.delayed_resume_work);
+		if (cancel_delayed_work_sync(&dev_priv->rps.delayed_resume_work))
+			intel_runtime_pm_put(dev_priv);
+
 		cancel_work_sync(&dev_priv->rps.work);
 		mutex_lock(&dev_priv->rps.hw_lock);
 		if (IS_VALLEYVIEW(dev))