drm/tegra: Add PRIME support

Implement very basic PRIME support. This currently only works with
buffers that are contiguous in memory and will refuse to import any
physically non-contiguous buffers.

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

diff --cc drivers/gpu/drm/tegra/drm.c
Simple merge

diff --cc drivers/gpu/drm/tegra/gem.c
Simple merge

diff --cc drivers/gpu/drm/tegra/gem.h
Simple merge