drm/tegra: gem: Correct iommu_map_sg() error checking

iommu_map_sg() doesn't return a error value, but a size of the requested
IOMMU mapping or zero in case of error.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>