drm/tegra: hdmi: Setup audio only if configured

The audio configuration is only valid if the HDMI codec has been
properly set up. Do not attempt to set up audio before that happens
because it causes a division by zero.

Note that this is only problematic on Tegra20 and Tegra30. Later chips
implement the division instructions which return zero when dividing by
zero and don't throw an exception.

Fixes: db5adf4d ("drm/tegra: hdmi: Fix audio to work with any pixel clock rate")
Reported-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>