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>