drm/sun4i: Add missing drm_atomic_helper_shutdown at driver unbind

A call to drm_atomic_helper_shutdown is required to properly release
the internal references taken by the core and avoid warnings about
leaking objects. Add it since it was missing.

Fixes: 9026e0d1

 ("drm: Add Allwinner A10 Display Engine support")
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190418132727.5128-2-paul.kocialkowski@bootlin.com