drm/mediatek: 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. 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 comes straight out of the kernel doc "driver
instance overview" in drm_drv.c.

This driver users the component model and shutdown happens in the base
driver. The "drvdata" for this driver will always be valid if
shutdown() is called and as of commit 2a073968


("drm/atomic-helper: drm_atomic_helper_shutdown(NULL) should be a
noop") we don't need to confirm that "drm" is non-NULL.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Fei Shao <fshao@chromium.org>
Tested-by: Fei Shao <fshao@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240611102744.v2.1.I2b014f90afc4729b6ecc7b5ddd1f6dedcea4625b@changeid