drm/i915/icl: Gate clocks for DSI

As per BSPEC, depending on the DSI transcoder being used,
DDI clock for the associated port should be gated. This
patch does the same.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e032bc3d8fff91b8c2631c73121268214615a7e8.1543500286.git.jani.nikula@intel.com