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