ALSA: hda: use last DAC defined for hp_pin

Patch allows the last DAC in the dac_nids for the hp_nid if there is an
available one this isn't in use by a line_out entry or if hp_nid isn't already
defined. This solves the issues with the 'Headphone Playback' mixer
controls on the 92hd73xxx branch and possibly others.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>