drm/imx/ipuv3: Call drm_atomic_helper_shutdown() at shutdown/unbind 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
and at driver unbind 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 and at driver remove (or unbind) time comes
straight out of the kernel doc "driver instance overview" in
drm_drv.c.

A few notes about this fix:
- When adding drm_atomic_helper_shutdown() to the unbind path, I added
  it after drm_kms_helper_poll_fini() since that's when other drivers
  seemed to have it.
- Technically with a previous patch, ("drm/atomic-helper:
  drm_atomic_helper_shutdown(NULL) should be a noop"), we don't
  actually need to check to see if our "drm" pointer is NULL before
  calling drm_atomic_helper_shutdown(). We'll leave the "if" test in,
  though, so that this patch can land without any dependencies. It
  could potentially be removed later.
- This patch also makes sure to set the drvdata to NULL in the case of
  bind errors to make sure that shutdown can't access freed data.
Suggested-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901164111.RFT.13.I0a9940ff6f387d6acf4e71d8c7dbaff8c42e3aaa@changeid

drivers/gpu/drm/imx/ipuv3/imx-drm-core.c

@@ -257,6 +257,7 @@ static int imx_drm_bind(struct device *dev)
 	drm_kms_helper_poll_fini(drm);
 	component_unbind_all(drm->dev, drm);
 err_kms:
+	dev_set_drvdata(dev, NULL);
 	drm_dev_put(drm);
 
 	return ret;
@@ -269,6 +270,7 @@ static void imx_drm_unbind(struct device *dev)
 	drm_dev_unregister(drm);
 
 	drm_kms_helper_poll_fini(drm);
+	drm_atomic_helper_shutdown(drm);
 
 	component_unbind_all(drm->dev, drm);
 
@@ -297,6 +299,11 @@ static void imx_drm_platform_remove(struct platform_device *pdev)
 	component_master_del(&pdev->dev, &imx_drm_ops);
 }
 
+static void imx_drm_platform_shutdown(struct platform_device *pdev)
+{
+	drm_atomic_helper_shutdown(platform_get_drvdata(pdev));
+}
+
 #ifdef CONFIG_PM_SLEEP
 static int imx_drm_suspend(struct device *dev)
 {
@@ -324,6 +331,7 @@ MODULE_DEVICE_TABLE(of, imx_drm_dt_ids);
 static struct platform_driver imx_drm_pdrv = {
 	.probe		= imx_drm_platform_probe,
 	.remove_new	= imx_drm_platform_remove,
+	.shutdown	= imx_drm_platform_shutdown,
 	.driver		= {
 		.name	= "imx-drm",
 		.pm	= &imx_drm_pm_ops,