drm/rockchip: shutdown drm subsystem on shutdown

As explained by Robin Murphy:
> the IOMMU shutdown disables paging, so if the VOP is still
> scanning out then that will result in whatever IOVAs it was using now going
> straight out onto the bus as physical addresses.

We had a more radical approach before in commit
7f3ef5de ("drm/rockchip: Allow driver to be shutdown on reboot/kexec")
but that resulted in new warnings and oopses on shutdown on rk3399
chromeos devices.

So second try is resurrecting Vicentes shutdown change which should
achieve the same result but in a less drastic way.

Fixes: 63238173 ("Revert "drm/rockchip: Allow driver to be shutdown on reboot/kexec"")
Cc: Jeffy Chen <jeffy.chen@rock-chips.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Brian Norris <briannorris@chromium.org>
Cc: Doug Anderson <dianders@chromium.org>
Cc: stable@vger.kernel.org
Suggested-by: JeffyChen <jeffy.chen@rock-chips.com>
Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Vicente Bergas <vicencb@gmail.com>
[adapted commit message to explain the history]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Brian Norris <briannorris@chromium.org>
Tested-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190402113753.10118-1-heiko@sntech.de

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -448,6 +448,14 @@ static int rockchip_drm_platform_remove(struct platform_device *pdev)
 	return 0;
 }
 
+static void rockchip_drm_platform_shutdown(struct platform_device *pdev)
+{
+	struct drm_device *drm = platform_get_drvdata(pdev);
+
+	if (drm)
+		drm_atomic_helper_shutdown(drm);
+}
+
 static const struct of_device_id rockchip_drm_dt_ids[] = {
 	{ .compatible = "rockchip,display-subsystem", },
 	{ /* sentinel */ },
@@ -457,6 +465,7 @@ MODULE_DEVICE_TABLE(of, rockchip_drm_dt_ids);
 static struct platform_driver rockchip_drm_platform_driver = {
 	.probe = rockchip_drm_platform_probe,
 	.remove = rockchip_drm_platform_remove,
+	.shutdown = rockchip_drm_platform_shutdown,
 	.driver = {
 		.name = "rockchip-drm",
 		.of_match_table = rockchip_drm_dt_ids,