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.

Fixes: c4755fb9 ("drm/tegra: Add Tegra186 display hub support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>