sound: via82xx: deactivate DXS controls of inactive streams

Activate the DXS volume controls only when the corresponding stream is
being used.  This makes the behaviour consistent with the other drivers
that have per-stream volume controls.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>