ALSA: HDA VIA: Fix 2nd S/PDIF out function

As it seems, the recently-sent patch for the 2nd S/PDIF (HDMI) output
is not working with alsa-kernel 1.0.18rc3.

This patch makes it work by
* activating the second S/PDIF output pin in the pin config
* consolidating the dig_playback_pcm_prepare() with extra_dig_pcm_prepare()
  functions
* remove the need for an extra hda_pcm_stream structure and rather represents
  the second digital output as substream within the primary S/PDIF digital out
  stream.

Signed-off-by: Logan Li <LoganLi@viatech.com.cn>
Signed-off-by: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>