drm/tegra: vic: Fix build warning when CONFIG_PM=n

drivers/gpu/drm/tegra/vic.c:326:12: error: ‘vic_runtime_suspend’ defined but not used [-Werror=unused-function]
 static int vic_runtime_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/tegra/vic.c:292:12: error: ‘vic_runtime_resume’ defined but not used [-Werror=unused-function]
 static int vic_runtime_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~

Mark it as __maybe_unused.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>