drm/tegra: Implement new UAPI

Implement the non-submission parts of the new UAPI, including
channel management and memory mapping. The UAPI is under the
CONFIG_DRM_TEGRA_STAGING config flag for now.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>