drm: renesas: shmobile: 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.
This is important because drm_atomic_helper_shutdown() will cause
panels to get disabled cleanly which may be important for their power
sequencing.  Future changes will remove any custom powering off in
individual panel drivers so the DRM drivers need to start getting this
right.

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

[geert: shmob_drm_remove() already calls drm_atomic_helper_shutdown]
Suggested-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20230901164111.RFT.15.Iaf638a1d4c8b3c307a6192efabb4cbb06b195f15@changeid
[geert: s/drm_helper_force_disable_all/drm_atomic_helper_shutdown/]
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/17c6a5a668e5975f871b77fb1fca6711a0799d9e.1718176895.git.geert+renesas@glider.be

drivers/gpu/drm/renesas/shmobile/shmob_drm_drv.c

@@ -171,6 +171,13 @@ static void shmob_drm_remove(struct platform_device *pdev)
 	drm_kms_helper_poll_fini(ddev);
 }
 
+static void shmob_drm_shutdown(struct platform_device *pdev)
+{
+	struct shmob_drm_device *sdev = platform_get_drvdata(pdev);
+
+	drm_atomic_helper_shutdown(&sdev->ddev);
+}
+
 static int shmob_drm_probe(struct platform_device *pdev)
 {
 	struct shmob_drm_platform_data *pdata = pdev->dev.platform_data;
@@ -273,6 +280,7 @@ static const struct of_device_id shmob_drm_of_table[] __maybe_unused = {
 static struct platform_driver shmob_drm_platform_driver = {
 	.probe		= shmob_drm_probe,
 	.remove_new	= shmob_drm_remove,
+	.shutdown	= shmob_drm_shutdown,
 	.driver		= {
 		.name	= "shmob-drm",
 		.of_match_table = of_match_ptr(shmob_drm_of_table),