libdrm: Add NVIDIA Tegra support

Add the libdrm_tegra helper library to encapsulate Tegra-specific
interfaces to the DRM.

Furthermore, Tegra is added to the list of supported chips in the
modetest and vbltest programs.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>