tegra: Fix mmap() of GEM buffer objects

Store 64-bit offset values and use libdrm's built-in drm_mmap() function
instead of mmap() to ensure the full 64-bit offset is used.

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