drm/v3d: Ensure job pointer is set to NULL after job completion

In the Linux kernel, the following vulnerability has been resolved: drm/v3d: Ensure job pointer is set to NULL after job completion After a job completes, the corresponding pointer in the device must be set to NULL. Failing to do so triggers a warning when unloading the driver, as it appears the job is still active. To prevent this, assign the job pointer to NULL after completing the job, indicating the job has finished.