drm/i915: Don't display nonsensical values in i915_ddb_info on gen < 9

When playing around with debugfs and a HSW machine I noticed that we
were displaying some garbled value in i915_ddb_info. This debugfs file
is only meaningful for gen9+, so don't display anything on earlier
platforms.
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2743,6 +2743,9 @@ static int i915_ddb_info(struct seq_file *m, void *unused)
 	enum pipe pipe;
 	int plane;
 
+	if (INTEL_INFO(dev)->gen < 9)
+		return 0;
+
 	drm_modeset_lock_all(dev);
 
 	ddb = &dev_priv->wm.skl_hw.ddb;