drm/tegra: Prevent BOs from being freed during job submission

Since DRM IOCTL's are lockless, there is a chance that BOs could be
released while a job submission is in progress. To avoid that, keep the
GEM reference until the job has been pinned, part of which will be to
take another reference.

v2: remove redundant check and avoid memory leak

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