drm/meson: osd_afbcd: Add an exit callback to struct meson_afbcd_ops

Use this to simplify the driver shutdown. It will also come handy when
fixing the error handling in meson_drv_bind_master().

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Fixes: d1b5e41e ("drm/meson: Add AFBCD module driver")
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211230235515.1627522-2-martin.blumenstingl@googlemail.com