drm/tegra: Relicense public header under MIT

This file will eventually be exported to libdrm, where all the public
header files use the MIT license.

Reported-by: Erik Faye-Lund <kusmabite@gmail.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>