drm/mipi-dsi: Detach devices when removing the host

Whenever the MIPI-DSI host is unregistered, the code of
mipi_dsi_host_unregister() loops over every device currently found on that
bus and will unregister it.

However, it doesn't detach it from the bus first, which leads to all kind
of resource leaks if the host wants to perform some clean up whenever a
device is detached.

Fixes: 068a0023 ("drm: Add MIPI DSI bus support")
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>