ALSA: hda - add DP MST audio support

This patch adds the DP MST audio support on i915 platform and
it will enable dyn_pcm_assign feature.

DP MST supports several device entry on the same port and each
device entry can map to one pcm stream. For example, on i915,
there are 3 pins, and each pin has 3 device entries. This means
there should be 3x3 pcms. However, there is only 3 pipe lines in
i915. This means 3 pcms are actived at most at the same moment.
We will create 5 pcms (pin number + dev entry num - 1) in this case.
For the details, please refer commit a76056f2
("ALSA: hda - hdmi dynamically bind PCM to pin when monitor hotplug")

Each device entry is a virtual pin. It is described by pin_nid and dev_id
in struct hdmi_spec_per_pin.
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1484208294-8637-3-git-send-email-libin.yang@intel.com

sound/pci/hda/hda_codec.c

@@ -467,6 +467,10 @@ static int read_pin_defaults(struct hda_codec *codec)
 		pin->nid = nid;
 		pin->cfg = snd_hda_codec_read(codec, nid, 0,
 					      AC_VERB_GET_CONFIG_DEFAULT, 0);
+		/*
+		 * all device entries are the same widget control so far
+		 * fixme: if any codec is different, need fix here
+		 */
 		pin->ctrl = snd_hda_codec_read(codec, nid, 0,
 					       AC_VERB_GET_PIN_WIDGET_CONTROL,
 					       0);