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

drivers/gpu/drm/i915/intel_dsi.c

@@ -872,6 +872,11 @@ static void intel_dsi_pre_disable(struct intel_encoder *encoder,
 			I915_WRITE(MIPI_DEVICE_READY(port), 0);
 	}
 
+	/*
+	 * According to the spec we should send SHUTDOWN before
+	 * MIPI_SEQ_DISPLAY_OFF only for v3+ VBTs, but field testing
+	 * has shown that the v3 sequence works for v2 VBTs too
+	 */
 	if (is_vid_mode(intel_dsi)) {
 		/* Send Shutdown command to the panel in LP mode */
 		for_each_dsi_port(port, intel_dsi->ports)