drm/exynos: 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 <[e-mail]>
Reviewed-by: Maxime Ripard <[e-mail]>
Signed-off-by: Douglas Anderson <[e-mail]>
Tested-by: Marek Szyprowski <[e-mail]>
Reviewed-by: Marek Szyprowski <[e-mail]>
Signed-off-by: Inki Dae <[e-mail]>

drivers/gpu/drm/exynos/exynos_drm_drv.c

···
 	drm_mode_config_cleanup(drm);
 	exynos_drm_cleanup_dma(drm);
 	kfree(private);
+	dev_set_drvdata(dev, NULL);
 err_free_drm:
 	drm_dev_put(drm);
 
···
 	drm_dev_unregister(drm);
 
 	drm_kms_helper_poll_fini(drm);
+	drm_atomic_helper_shutdown(drm);
 
 	component_unbind_all(drm->dev, drm);
 	drm_mode_config_cleanup(drm);
···
 	return 0;
 }
 
+static void exynos_drm_platform_shutdown(struct platform_device *pdev)
+{
+	struct drm_device *drm = platform_get_drvdata(pdev);
+
+	if (drm)
+		drm_atomic_helper_shutdown(drm);
+}
+
 static struct platform_driver exynos_drm_platform_driver = {
 	.probe	= exynos_drm_platform_probe,
 	.remove	= exynos_drm_platform_remove,
+	.shutdown = exynos_drm_platform_shutdown,
 	.driver	= {
 		.name	= "exynos-drm",
 		.pm	= &exynos_drm_pm_ops,