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>