drm: Prevent NULL deref in drm_name_info()

If a driver does not have a parent, or never sets the unique name for
itself, then we may proceed to chase a NULL dereference through
debugfs/.../name.

Testcase: igt/vgem_basic/debugfs
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466448813-23340-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_info.c

@@ -51,17 +51,16 @@ int drm_name_info(struct seq_file *m, void *data)
 	struct drm_minor *minor = node->minor;
 	struct drm_device *dev = minor->dev;
 	struct drm_master *master = minor->master;
-	if (!master)
-		return 0;
-
-	if (master->unique) {
-		seq_printf(m, "%s %s %s\n",
-			   dev->driver->name,
-			   dev_name(dev->dev), master->unique);
-	} else {
-		seq_printf(m, "%s %s\n",
-			   dev->driver->name, dev_name(dev->dev));
-	}
+
+	seq_printf(m, "%s", dev->driver->name);
+	if (dev->dev)
+		seq_printf(m, " dev=%s", dev_name(dev->dev));
+	if (master && master->unique)
+		seq_printf(m, " master=%s", master->unique);
+	if (dev->unique)
+		seq_printf(m, " unique=%s", dev->unique);
+	seq_printf(m, "\n");
+
 	return 0;
 }