drm: remove outdated doc TODO for subconnector property

This is already documented under "standard connector properties".

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230227122522.117580-1-contact@emersion.fr