drm/i915: fix unbalanced GT powersave enable / disable calls

Atm, we call intel_gt_powersave_enable() for GEN6 and GEN7 but disable
it for everything starting from GEN6. This is a problem in case of BDW.
Since I don't have a BDW to test if RC6 works properly, just keep it
disabled for now and fix only the disable function.
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -4516,7 +4516,7 @@ void intel_disable_gt_powersave(struct drm_device *dev)
 	if (IS_IRONLAKE_M(dev)) {
 		ironlake_disable_drps(dev);
 		ironlake_disable_rc6(dev);
-	} else if (INTEL_INFO(dev)->gen >= 6) {
+	} else if (IS_GEN6(dev) || IS_GEN7(dev)) {
 		cancel_delayed_work_sync(&dev_priv->rps.delayed_resume_work);
 		cancel_work_sync(&dev_priv->rps.work);
 		mutex_lock(&dev_priv->rps.hw_lock);