Add the missing platform_device_unregister() before return
from mtk_mmsys_probe() in the error handling case.

Fixes: 667c7692 ("soc / drm: mediatek: Fix mediatek-drm device probing")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20200506141317.119537-1-weiyongjun1@huawei.comSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>