drm/tegra: submit: Add missing pm_runtime_mark_last_busy()

Runtime PM auto-suspension doesn't work without pm_runtime_mark_last_busy(),
add it.

Cc: <stable@vger.kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>