drm/mediatek: Optimize functions which do not need to return

Function mtk_hdmi_aud_set_input always return 0, no need to
keep the return value. Functions mtk_hdmi_aud_enable_packet &
mtk_hdmi_aud_on_off_hw_ncts are the same, these two functions
just call next functions. Maybe it`s a bit better to just call
the inner function.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>