The subconnector property was created by drm_mode_create_tv_properties(),
but wasn't exposed to the userspace through the generic
atomic_get/set_property implementation, and wasn't stored in any generic
state structure.

Let's solve this.
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v4-6-60d38873f782@cerno.techSigned-off-by: Maxime Ripard <maxime@cerno.tech>