ALSA: hda/realtek: Reduce the Headphone static noise on XPS 9350/9360

Headphone on XPS 9350/9360 produces a background white noise. The The
noise level somehow correlates with "Headphone Mic Boost", when it sets
to 1 the noise disappears. However, doing this has a side effect, which
also decreases the overall headphone volume so I didn't send the patch
upstream.

The noise was bearable back then, but after commit 717f43d8 ("ALSA:
hda/realtek - Update headset mode for ALC256") the noise exacerbates to
a point it starts hurting ears.

So let's use the workaround to set "Headphone Mic Boost" to 1 and lock
it so it's not touchable by userspace.

Fixes: 717f43d8

 ("ALSA: hda/realtek - Update headset mode for ALC256")
BugLink: https://bugs.launchpad.net/bugs/1654448
BugLink: https://bugs.launchpad.net/bugs/1845810
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Link: https://lore.kernel.org/r/20191003043919.10960-1-kai.heng.feng@canonical.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>