drm/mediatek: exit earlier if failed to register audio driver

Exits earlier if register_audio_driver() returns errors.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Acked-by: CK Hu <ck.hu@mediatek.com>
Link: https://lore.kernel.org/r/20200206102509.1.Ieba8d422486264eb7aaa3aa257620a1b0c74c6db@changeid


Signed-off-by: Mark Brown <broonie@kernel.org>