ALSA: x86: hdmi: Add single_port option for compatible behavior

The recent support for the multiple PCM devices allowed user to use
multiple HDMI/DP outputs, but at the same time, the PCM stream
assignment has been changed, too.  Due to that, the former PCM#0
(there was only one stream in the past) is likely assigned to a
different one (e.g. PCM#2), and it ends up with the regression when
user sticks with the fixed configuration using the device#0.

Although the multiple monitor support shouldn't matter when user
deploys the backend like PulseAudio that checks the jack detection
state, the behavior change isn't always acceptable for some users.

As a mitigation, this patch introduces an option to switch the
behavior back to the old-good-days: when the new option,
single_port=1, is passed, the driver creates only a single PCM device,
and it's assigned to the first connected one, like the earlier
versions did.  The option is turned off as default still to support
the multiple monitors.

Fixes: 8a2d6ae1 ("ALSA: x86: Register multiple PCM devices for the LPE audio card")
Reported-and-tested-by: Hubert Mantel <mantel@metadox.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>