ALSA: HDA patch_via.c: Independent DAC for headphone

This mode allows an output stream to have two substreams, one for the
speakers and one for the headphone.  Each of the substreams has independent
PCM data and uses a different DAC.
Signed-off-by: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>