drm/i915/dsi: Move most things from .disable() into .post_disable() on icl+

encoder->disable() is supposed to happen before the pipe/transcoder
gets disabled. The icl+ DSI code screwed that up and put most things
(including the transcoder disable itself) into  encoder->disable().
Follow the common rules and hoist most things into the
encoder->post_disable() hook.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230608203057.23759-9-ville.syrjala@linux.intel.com