drm/tegra: gem: Do not try to dereference ERR_PTR()

When mapping the DMA-BUF attachment fails, map->sgt will be an ERR_PTR-
encoded error code and the cleanup code would try to free that memory,
which obviously would fail.

Zero out that pointer after extracting the error code when this happens
so that kfree() can do the right thing.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>