drm/tegra: Fix NULL deref in debugfs/iova

When IOMMU is off, ->mm_lock is not initialized and ->mm is NULL.
Signed-off-by: NMichał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/drm.c

@@ -1033,9 +1033,11 @@ static int tegra_debugfs_iova(struct seq_file *s, void *data)
 	struct tegra_drm *tegra = drm->dev_private;
 	struct drm_printer p = drm_seq_file_printer(s);
 
+	if (tegra->domain) {
 		mutex_lock(&tegra->mm_lock);
 		drm_mm_print(&tegra->mm, &p);
 		mutex_unlock(&tegra->mm_lock);
+	}
 
 	return 0;
 }