Our SDVO audio support is pretty bogus. We can't push audio over the
SDVO bus, so trying to enable audio in the SDVO control register doesn't
do anything. In fact it looks like the SDVO encoder will always mix in
the audio coming over HDA, and there's no (at least documented) way to
disable that from our side. So HDMI audio does work currently on gen4
but only by luck really. On gen3 it got broken by the referenced commit.
And what has always been missing on every platform is the ELD.

To pass the ELD to the audio driver we need to write it to magic buffer
in the SDVO encoder hardware which then gets pulled out via HDA in the
other end. Ie. pretty much the same thing we had for native HDMI before
we started to just pass the ELD between the drivers. This sort of
explains why we even have that silly hardware buffer with native HDMI.

$ cat /proc/asound/card0/eld#1.0
-monitor_present		0
-eld_valid		0
+monitor_present		1
+eld_valid		1
+monitor_name		LG TV
+connection_type		HDMI
+...

This also fixes our state readout since we can now query the SDVO
encoder about the state of the "ELD valid" and "presence detect"
bits. As mentioned those don't actually control whether audio
gets sent over the HDMI cable, but it's the best we can do. And with
the state checker appeased we can re-enable HDMI audio for gen3.

Cc: stable@vger.kernel.org
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: zardam@gmail.com
Tested-by: zardam@gmail.com
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108976
Fixes: de44e256b92c ("drm/i915/sdvo: Shut up state checker with hdmi cards on gen3")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190409144054.24561-3-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>
(cherry picked from commit dc49a56bd43bb04982e64b44436831da801d0237)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>