ALSA: hda/realtek - Fix missing volume controls with ALC260

ALSA: hda/realtek - Fix missing volume controls with ALC260

ALC260 has multiple mixer widgets connected to the shared DAC, but the
driver currently doesn't check this possibility and ignores when the DAC
is shared with others.  This resulted in the silent output from some
routes because of lack of the amp setup.

This patch adds the workaround for it by checking the route even with the
shared DAC, but also checking the conflict with the existing control for
the very same widget NID.

Reference: https://bugzilla.novell.com/show_bug.cgi?id=726812

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