drm/i915/icl: Define missing bitfield for shortplug reg

This patch define missing bitfield for shortplug ctl ddi
register which will be used for ICL DSI GPIO programming.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/4b9feb75eb6c303556b91c8a23c505a4593a99a1.1543500286.git.jani.nikula@intel.com