drm/scdc: Document hotplug gotchas

There's some interactions between the SCDC setup and the disconnection /
reconnection of displays. Let's document it and a solution.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220829134731.213478-9-maxime@cerno.tech