drm/bridge: Document the expected behaviour of DSI host controllers

The exact behaviour of DSI host controllers is not specified,
therefore define it.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/20221205173328.1395350-7-dave.stevenson@raspberrypi.com


Signed-off-by: Maxime Ripard <maxime@cerno.tech>