drm/nva3/disp: Fix HDMI audio regression

This is the nva3 counterpart to commit beba44b1 (drm/nv84/disp: Fix
HDMI audio regression). The regression happened as a result of
refactoring in commit 8e9e3d2d (drm/nv84/disp: move hdmi control into
core).

Reported-and-tested-by: Max Baldwin <archerseven@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org