drm/tegra: sor: Mark PM functions as __maybe_unused

Without CONFIG_PM, some functions cause harmless warnings:

drivers/gpu/drm/tegra/sor.c:3984:12: error: 'tegra_sor_resume' defined but not used [-Werror=unused-function]
 static int tegra_sor_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~
drivers/gpu/drm/tegra/sor.c:3970:12: error: 'tegra_sor_suspend' defined but not used [-Werror=unused-function]
 static int tegra_sor_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~

Mark these as __maybe_unused so the compiler can drop them
silently.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>