drm/tegra: Add call to put_pid()

In the Linux kernel, the following vulnerability has been resolved: drm/tegra: Add call to put_pid() Add a call to put_pid() corresponding to get_task_pid(). host1x_memory_context_alloc() does not take ownership of the PID so we need to free it here to avoid leaking. [mperttunen@nvidia.com: reword commit message]