ALSA: x86: Enable keep-link feature

This patch enables the "keep-link" feature experimentally.  It's a
feature where the device keeps the link and sending the silent output
even after the PCM device is closed.  Then the receiver will be
resumed quickly once when a PCM is opened and a stream is sent again.

The stream link is turned off when the device goes to autosuspend or
manual suspend.

This new keep-link feature itself is controlled via a new module
option, keep_link.  You can turn it on/off, again, via sysfs like:

  echo 0 > /sys/module/snd_hdmi_lpe_audio/parameters/keep_link

As default, the feature is turned on.

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