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