ALSA: hda - Enable i915 binding for Intel gen4 HDMI/DP

This patch adds the support of i915 audio handling to some gen4 chips,
Eaglelake and Cantiga.  There are still a few Intel codecs left with
patch_generic_hdmi(): CederTrail is with powervr, so it's not bound
with i915, and Crestline isn't handled in i915 audio.  Bearlake
doesn't appear in the market actually, so safer to leave it out.

In the current patch, we assume a single pin/converter pair with the
fixed digital port mapping.  This might need to be fixed later.

Signed-off-by: Takashi Iwai <tiwai@suse.de>