drm/tegra: Mark Tegra186 display hub PM functions __maybe_unused
The newly introduced driver has optional suspend/resume functions,
causing a warning when CONFIG_PM is disabled:
drivers/gpu/drm/tegra/hub.c:749:12: error: 'tegra_display_hub_resume' defined but not used [-Werror=unused-function]
drivers/gpu/drm/tegra/hub.c:733:12: error: 'tegra_display_hub_suspend' defined but not used [-Werror=unused-function]
This marks them __maybe_unused to shut up the warnings.