ALSA: hda - Use the primary DAC for all aamix outputs

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>