Add get_eld audio component for i915/HD-audio

Here are the patchset to add get_eld op to audio component for
communicating more directly between i915 and HD-audio.

Currently, the HDMI/DP audio status and ELD are notified and obtained
via the hardware-level communication over HD-audio unsolicited event
and verbs although the graphics driver holds the exactly same
information.  As we already have a notification via audio component,
this is another step forward; namely, the audio driver may fetch
directly the audio status and ELD via the new component op.

The commits are based on Dave's latest drm-next branch.
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