drm/armada: Call drm_atomic_helper_shutdown() at shutdown time

Based on grepping through the source code this driver appears to be
missing a call to drm_atomic_helper_shutdown() at system shutdown
time. Among other things, this means that if a panel is in use that it
won't be cleanly powered off at system shutdown time.

The fact that we should call drm_atomic_helper_shutdown() in the case
of OS shutdown/restart comes straight out of the kernel doc "driver
instance overview" in drm_drv.c.

This driver was fairly easy to update. The drm_device is stored in the
drvdata so we just have to make sure the drvdata is NULL whenever the
device is not bound. To make things simpler,
drm_atomic_helper_shutdown() has been modified to consider a NULL
drm_device as a noop in the patch ("drm/atomic-helper:
drm_atomic_helper_shutdown(NULL) should be a noop").
Suggested-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901164111.RFT.1.I3d5598bd73a59b5ded71430736c93f67dc5dea61@changeid

drivers/gpu/drm/armada/armada_drv.c

@@ -148,6 +148,7 @@ static int armada_drm_bind(struct device *dev)
  err_kms:
 	drm_mode_config_cleanup(&priv->drm);
 	drm_mm_takedown(&priv->linear);
+	dev_set_drvdata(dev, NULL);
 	return ret;
 }
 
@@ -166,6 +167,7 @@ static void armada_drm_unbind(struct device *dev)
 
 	drm_mode_config_cleanup(&priv->drm);
 	drm_mm_takedown(&priv->linear);
+	dev_set_drvdata(dev, NULL);
 }
 
 static void armada_add_endpoints(struct device *dev,
@@ -230,6 +232,11 @@ static int armada_drm_remove(struct platform_device *pdev)
 	return 0;
 }
 
+static void armada_drm_shutdown(struct platform_device *pdev)
+{
+	drm_atomic_helper_shutdown(platform_get_drvdata(pdev));
+}
+
 static const struct platform_device_id armada_drm_platform_ids[] = {
 	{
 		.name		= "armada-drm",
@@ -243,6 +250,7 @@ MODULE_DEVICE_TABLE(platform, armada_drm_platform_ids);
 static struct platform_driver armada_drm_platform_driver = {
 	.probe	= armada_drm_probe,
 	.remove	= armada_drm_remove,
+	.shutdown = armada_drm_shutdown,
 	.driver	= {
 		.name	= "armada-drm",
 	},