drm/nouveau/device: audit and version NV_DEVICE class

The full object interfaces are about to be exposed to userspace, so we
need to check for any security-related issues and version the structs
to make it easier to handle any changes we may need in the future.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/core/engine/disp/nv50.c b/drivers/gpu/drm/nouveau/core/engine/disp/nv50.c
index 5311f0f..ca8ce9c 100644
--- a/drivers/gpu/drm/nouveau/core/engine/disp/nv50.c
+++ b/drivers/gpu/drm/nouveau/core/engine/disp/nv50.c
@@ -990,7 +990,7 @@
 	int ret = -EBUSY;
 
 	/* no context needed for channel objects... */
-	if (nv_mclass(parent) != NV_DEVICE_CLASS) {
+	if (nv_mclass(parent) != NV_DEVICE) {
 		atomic_inc(&parent->refcount);
 		*pobject = parent;
 		return 1;