When setting up the aamix output paths, use the primary DAC instead of
the individual DAC for each output as default.  Otherwise multiple
DACs will be turned on for a single aamix widget, which results in
doubly or more volumes, because the duplicated signals will be sent
through all these DACs for a single stream.

Signed-off-by: Takashi Iwai <tiwai@suse.de>