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>

drivers/gpu/drm/tegra/submit.c

@@ -680,8 +680,7 @@ int tegra_drm_ioctl_channel_submit(struct drm_device *drm, void *data,
 		kfree(job_data->used_mappings);
 	}
 
-	if (job_data)
-		kfree(job_data);
+	kfree(job_data);
 put_bo:
 	gather_bo_put(&bo->base);
 unlock: