ALSA: hda/realtek - Simplify alc_auto_is_dac_reachable()

Use the helper function snd_hda_get_conn_index() instead of open
codes.  This also improves the detection of some routes to DAC on
ALC260 (although the difference doesn't influence on the end
results of the mapping).

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