drm/atomic-helper: drm_atomic_helper_shutdown(NULL) should be a noop

As with other places in the Linux kernel--kfree(NULL) being the most
famous example--it's convenient to treat being passed a NULL argument
as a noop in cleanup functions. Let's make
drm_atomic_helper_shutdown() work like this.

This is convenient for DRM devices that use the "component" model. On
these devices we want shutdown to be a noop if the bind() call of the
component hasn't been called yet. As long as drivers are careful to
make sure the drvdata is NULL whenever the driver is not bound then we
can just do a simple call to drm_atomic_helper_shutdown() with the
drvdata at shutdown time.

Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230901163944.RFT.1.I906acd535bece03b6671d97c2826c6f0444f4728@changeid