Allow codec driver register callback function for plug event.

The callback registration flow:
dw-hdmi <--- hw-hdmi-i2s-audio <--- hdmi-codec

dw-hdmi-i2s-audio implements hook_plugged_cb op
so codec driver can register the callback.

dw-hdmi exports a function dw_hdmi_set_plugged_cb so platform device
can register the callback.

When connector plug/unplug event happens, report this event using the
callback.

Make sure that audio and drm are using the single source of truth for
connector status.
Signed-off-by: Cheng-Yi Chiang <cychiang@chromium.org>
Link: https://lore.kernel.org/r/20191028071930.145899-2-cychiang@chromium.orgSigned-off-by: Mark Brown <broonie@kernel.org>