drm/i915/dsi: Document the panel enable / disable sequences from the spec

Document the DSI panel enable / disable sequences from the spec,
for easy comparison between the code and the spec.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jani Nikula <jani.nikula@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/1488374106-4949-2-git-send-email-jani.nikula@intel.com