drm/i915/icl: Disable DDI function

This patch disables DDI function by writing to
TRANS_DDI_FUNC_CTL registers of DSI ports as part
of DSI disable sequence.

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/b0088d643247135ba96943fa14625f5a43f43633.1540900289.git.jani.nikula@intel.com