drm/i915: Support not having an init clock gating function defined

When enabling new platforms, we may not have any W/A to apply,
especially that, now, a bunch of them have to be done from the ring.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -6386,7 +6386,8 @@ void intel_init_clock_gating(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
-	dev_priv->display.init_clock_gating(dev);
+	if (dev_priv->display.init_clock_gating)
+		dev_priv->display.init_clock_gating(dev);
 }
 
 void intel_suspend_hw(struct drm_device *dev)