drm/tegra: hdmi: Detect DVI-only displays

Use EDID data to determine whether the display supports HDMI or DVI
only. The HDMI output used to assume to be connected to HDMI displays,
but that broke support for DVI displays that don't understand the
interspersed audio/other data.

To be on the safe side, default to DVI if no EDID data is available.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
[treding@nvidia.com: move detection to separate function]
Signed-off-by: Thierry Reding <treding@nvidia.com>