drm/device: Deprecate dev_private harder

We've had lots of conversions to embeddeding, but didn't stop using
->dev_private. Which defeats the point of this.
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200403135828.2542770-4-daniel.vetter@ffwll.ch

include/drm/drm_device.h

@@ -88,9 +88,12 @@ struct drm_device {
 	/**
 	 * @dev_private:
 	 *
-	 * DRM driver private data. Instead of using this pointer it is
-	 * recommended that drivers use drm_dev_init() and embed struct
-	 * &drm_device in their larger per-device structure.
+	 * DRM driver private data. This is deprecated and should be left set to
+	 * NULL.
+	 *
+	 * Instead of using this pointer it is recommended that drivers use
+	 * drm_dev_init() and embed struct &drm_device in their larger
+	 * per-device structure.
 	 */
 	void *dev_private;