drm/i915/audio: add DOC comment describing HDA over HDMI/DP

v2: include the section in the drm docbook.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>