drm/tegra: gem: Return 64-bit offset for mmap(2)

On 64-bit targets, tegra_gem_mmap() only returns a partial offset to
userspace. As such, subsequent calls to mmap(2) may fail. Change the
arguments to use a 64-bit offset to fix this.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Erik Faye-Lund <kusmabite@gmail.com>
[treding@nvidia.com: tweak commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>