SOUT has many bits and need to be cleared before set new value.
Write only could do the clear, but for MOUT, it clears bits that
should not be cleared. So use a mask to reset only the needed bits.

this fixes HDMI issues on MT7623/BPI-R2 since 5.13

Fixes: 44014763 ("soc: mediatek: mmsys: Use an array for setting the routing registers")
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Link: https://lore.kernel.org/r/20210729070549.5514-1-linux@fw-web.deSigned-off-by: Matthias Brugger <matthias.bgg@gmail.com>