drm/i915/icl: Put DSI link in ULPS

As part of DSI disabling sequence, DSI link need to enter
in ULPS by writing into DSI_LP_MSG register. This patch
does the same using a wrapper function.

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/50812f4bd37f95c053bef7eef4a95e5da029546f.1540900289.git.jani.nikula@intel.com