ALSA: hda/realtek - Fix dock line-out volume on Dell Precision 7520

ALSA: hda/realtek - Fix dock line-out volume on Dell Precision 7520

The dock line-out pin (NID 0x17 of ALC3254 codec) on Dell Precision
7520 may route to three different DACs, 0x02, 0x03 and 0x06.  The
first two DACS have the volume amp controls while the last one
doesn't.  And unfortunately, the auto-parser assigns this pin to DAC3,
resulting in the non-working volume control for the line out.

Fix it by disabling the routing to DAC3 on the corresponding pin.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=199029
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>