drm/i915/dsi: Document always using v3 SHUTDOWN / MIPI_SEQ_DISPLAY_OFF order

According to the spec for v2 VBTs we should call MIPI_SEQ_DISPLAY_OFF
before sending SHUTDOWN, where as for v3 VBTs we should send SHUTDOWN
first.

Since the v2 order has known issues, we use the v3 order everywhere,
add a comment documenting this.

Signed-off-by: Hans de Goede <hdegoede@redhat.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/1488374106-4949-8-git-send-email-jani.nikula@intel.com