drm/i915: Cannot set clock gating under UMS

The clock gating functions are only assigned under KMS, so don't try
to call them under UMS.
Signed-off-by: Keith Packard <keithp@keithp.com>
Tested-by: Justin P. Mattock <justinmattock@gmail.com>

drivers/gpu/drm/i915/i915_suspend.c

@@ -871,7 +871,8 @@ int i915_restore_state(struct drm_device *dev)
 	}
 	mutex_unlock(&dev->struct_mutex);
 
-	intel_init_clock_gating(dev);
+	if (drm_core_check_feature(dev, DRIVER_MODESET))
+		intel_init_clock_gating(dev);
 
 	if (IS_IRONLAKE_M(dev)) {
 		ironlake_enable_drps(dev);