Begin to support dynamic PCM assignment to pin in
hdmi audio driver.

This means PCM will not be statically bound with pin.
When there is a monitor connected, the corresponding pin
will try to find a proper PCM to bind. When the monitor
is disconnected, the corresponding pin will unbind
the PCM. This helps to reduce the PCM number when there
are many pins (device entries in DP MST mode) and only
a few of them work at the same time.

This patch adds the pcm member in struct hdmi_spec_per_pin.
When PCM is dynamically bound to the pin, the member pcm
will pointer to the corresponding pcm_rec[] in hdmi_spec,
which means the hda_pcm is bound to the pin.
Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>