drm/tegra: uapi: Fix wrong mapping end address in case of disabled IOMMU

All jobs are failing on Tegra20 because it doesn't use IOMMU and mapping
function uses size of mapping that is zero instead of BO size, fix it.

Fixes: d7c591bc ("drm/tegra: Implement new UAPI")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>