ALSA: hda - Don't add aa-mix for VIA surrounds

Since we now route the front DAC via aa-mix widget, adding the aa-mix
to surrounds will result in a mix-up of both front and surround PCM
signals.  For avoiding this, the aa-mix routes have to be disabled
for surround paths.

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