drm: tegra: Add HDMI support

This commit adds support for the HDMI output on the Tegra20 SoC. Only
one such output is available, but it can be driven by either of the two
display controllers.

A lot of work on this patch has been contributed by NVIDIA's Mark Zhang
<markz@nvidia.com> and many other people at NVIDIA were very helpful in
getting the HDMI support and surrounding infrastructure to work.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Tested-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Zhang <markz@nvidia.com>
Reviewed-by: Mark Zhang <markz@nvidia.com>
Tested-by: Mark Zhang <markz@nvidia.com>
Tested-and-acked-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>