ALSA: hda/realtek - Add output path parser

Add the output path parser to Realtek codec driver as we already have
in patch_via.c.  The nid_path struct represents the complete output
path from a DAC to a pin.  The alc_spec contains an array of these
paths, and a new path is added at each time when a new DAC is
assigned.

So far, this path list is used only in limited codes: namely in this
patch, only alc_is_dac_already_used() checks the list instead of dac
arrays in all possible outputs.  In the later development, the path
list will be referred from more places, such as the mixer control
assignment / check, the mute/unmute of active routes, etc.

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