drm/i915/icl: Define DSI mode ctl register

This patch defines DSI IO mode control register and it's bits
used while enabling IO power for DSI.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1530798591-2077-4-git-send-email-madhav.chauhan@intel.com