drm/i915: add back checking for i915_disable_power_well

In

commit 6efdf354ddb186c6604d1692075421e8d2c740e9
Author: Imre Deak <imre.deak@intel.com>
Date:   Wed Oct 16 17:25:52 2013 +0300

the check for i915_disable_power_well flag was removed by overlook,
so add it back now.

Reported-by: Paulo Zanoni <paulo.zanoni@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -5592,7 +5592,7 @@ static void __intel_power_well_put(struct drm_device *dev,
 				   struct i915_power_well *power_well)
 {
 	WARN_ON(!power_well->count);
-	if (!--power_well->count)
+	if (!--power_well->count && i915_disable_power_well)
 		__intel_set_power_well(dev, false);
 }