ALSA: hda - hdmi playback without monitor in dynamic pcm bind mode

Pulseaudio requires open pcm successfully when probing.

This patch handles playback without monitor in dynamic pcm assignment
mode. It tries to open/prepare/close pcm successfully even there is
no pin bound to the PCM. On the meantime, it will try to find a proper
converter for the PCM.

As pcm is This patch introduces a pcm_lock in struct hdmi_spec.
This lock is used to protect:
1. the variables in struct hdmi_spec;
2. other variables shared for dynamic pcm assignment mode
3. device entry selection. As each device entry is represented by
   a separate struct struct hdmi_spec_per_pin, the lock in per_pin
   is not enough. Please see details below.

MST audio device entry operation:
1. select device entry on the pin
2. operate on the pin nid

Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>