drm/i915/dsi: Implement encoder->shutdown() for icl+

Plug in the encoder->shutdown() hook for icl+ DSI so that
we are guaranteed to respect the power cycle delay during
reboots and whatnot.

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/20230608203057.23759-8-ville.syrjala@linux.intel.com