ALSA: hda - Fix silent output regression with ALC861

The 3.1 kernel has a regression for ALC861 codec where no sound output
is heard with the default setup.  It's because the amps in DACs aren't
properly unmuted while the output mixers are assigned only to pins.

This patch fixes the missing initialization of DACs when no mixer is
assigned to them.

Tested-by: Andrea Iob <andrea_iob@yahoo.it>
Cc: <stable@kernel.org> [v3.1+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>