drm/i915: Add audio pin sense / ELD callback

This callback will be called by the i915 driver to notify the hda
driver that its HDMI information needs to be refreshed, i e,
that audio output is now available (or unavailable) - usually as a
result of a monitor being plugged in (or unplugged).

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Takashi Iwai <tiwai@suse.de>