ALSA: x86: intel_hdmi: add definitions and logic for DP audio

Imported from legacy patches

Note: the new code doesn't assume a modified ELD but
an explicit notification that DP is present. It appears
that the i915 code does change the ELD so we could use
the ELD-based tests to check for DP audio

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>