Now that everything is in place let's register a PCM device for
each port of the display engine. This will make it possible to
actually output audio to multiple displays at the same time. And
it avoids modesets on unrelated displays from clobbering up the
ELD and whatnot for the display currently doing the playback.

v2: Add a PCM per port instead of per pipe
v3: Fix off by one error with port numbers (Pierre-Louis)
    Fix .notify_audio_lpe() prototype (Pierre-Louis)

Cc: Takashi Iwai <tiwai@suse.de>
Cc: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170427160231.13337-12-ville.syrjala@linux.intel.comReviewed-by: Takashi Iwai <tiwai@suse.de>