(Re)enable DP/HDMI audio for RK3399 Gru

Merge series from Brian Norris <briannorris@chromium.org>:

This series fixes DP/HDMI audio for RK3399 Gru systems.

First, there was a regression with the switch to SPDIF. Patch 1 can be
taken separately as a regression fix if desired. But it's not quite so
useful (at least on Chrome OS systems) without the second part.

Second, jack detection was never upstreamed, because the hdmi-codec
dependencies were still being worked out when this platform was first
supported.

base-commit: e783362e