drm/nv84/disp: Fix HDMI audio regression

Code refactoring in commit 8e9e3d2d
(drm/nv84/disp: move hdmi control into core) disabled HDMI audio on my
nv84 by removing too much old code without adding it in the new one.
This patch adds the missing code within the new code layout resulting in
HDMI audio working again.
It should work on any HDMI head, but due to lacking ahrdware I could
only test the (1st) one.
It also might be possible that similar code is needed for nva3, which I
can't test.

Signed-off-by: Alexander Stein <alexander.stein@informatik.tu-chemnitz.de>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>