drm/tegra: hdmi: Fix audio to work with any pixel clock rate

The audio setting implementation was limited to a few specific pixel
clocks. This prevented HDMI audio from working on several test devices
as they need a pixel clock that is not supported by this implementation.

Fix this by implementing the algorithm provided in the TRM using fixed
point arithmetic. This allows the driver to cope with any sane pixel
clock rate.

Signed-off-by: Alban Bedel <alban.bedel@avionic-design.de>
[treding@nvidia.com: fix uninitialized variable warning]
Signed-off-by: Thierry Reding <treding@nvidia.com>