drm/tegra: submit: Remove pm_runtime_enabled() checks

Runtime PM is now universally available, make it mandatory by removing
the pm_runtime_enabled() checks.

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