When a machine has multiple speakers, we don't need to create the
controls for multi-ios.  Check the number of primary outputs beforehand.

Note that this workaround might not work always with new codecs in
future; this assumes that both speakers and multi-io jacks share the
same mixers/DACs.  If they are routed with different mixers, the
individual mixer controls should be needed.  But, so far, this doesn't
happen with the existing ALC codecs.
Signed-off-by: Takashi Iwai <tiwai@suse.de>