drm/i915/icl: Disable DSI ports

This patch disables both DSI ports by writing to
DDI_BUF_CTL registers as part of DSI encoder 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/be9a7dc1ed89ad38679c67ff1c1552f0b4604494.1540900289.git.jani.nikula@intel.com