drm/dp_helper: Mention the concurrency requirement hw_mutex

Drivers that allow concurrent access over multiple DP channels need to
provide additional locking, even though the hw_mutex field might
indicate otherwise. Clarify it in the documentation.
Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210616141529.630719-2-maxime@cerno.tech

include/drm/drm_dp_helper.h

@@ -1913,6 +1913,10 @@ struct drm_dp_aux {
 
 	/**
 	 * @hw_mutex: internal mutex used for locking transfers.
+	 *
+	 * Note that if the underlying hardware is shared among multiple
+	 * channels, the driver needs to do additional locking to
+	 * prevent concurrent access.
 	 */
 	struct mutex hw_mutex;