drm/i915/bxt: Disable device ready before shutdown command

Disable device ready before MIPI port shutdown command.
This helps to avoid mipi split screen issues.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Vidya Srinivas <vidya.srinivas@intel.com>
Reviewed-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486551058-22596-8-git-send-email-vidya.srinivas@intel.com