ALSA: hda - Fix loopback noise on Dell XPS 15

Dell XPS 15 (1028:05fe) gives the bad feedback noise from analog
loopback line even if the channels are muted.  The similar problem has
been seen on XPS 13, and it was fixed by simply disabling loopback.
The same fixup was confirmed to be applicable and fix the problem on
XPS 15, too.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=80821
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --cc sound/pci/hda/patch_realtek.c
Simple merge