drm/tegra: Remove redundant null checks before kfree

Fix the following coccicheck warning:
./drivers/gpu/drm/tegra/submit.c:689:2-7: WARNING:
NULL check before some freeing functions is not needed.

Signed-off-by: Yushan Zhou <katrinzhou@tencent.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>