drm/i915: Don't call describe_obj on NULL pointers

Reported-by: Pavel Roskin <proski@gnu.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38777
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1207,13 +1207,17 @@ static int i915_context_status(struct seq_file *m, void *unused)
 	if (ret)
 		return ret;
 
+	if (dev_priv->pwrctx) {
 		seq_printf(m, "power context ");
 		describe_obj(m, dev_priv->pwrctx);
 		seq_printf(m, "\n");
+	}
 
+	if (dev_priv->renderctx) {
 		seq_printf(m, "render context ");
 		describe_obj(m, dev_priv->renderctx);
 		seq_printf(m, "\n");
+	}
 
 	mutex_unlock(&dev->mode_config.mutex);