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: c4755fb9064f ("drm/tegra: Add Tegra186 display hub support")
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/drm/tegra/hub.c

···
 	return err;
 }
 
-static int tegra_display_hub_suspend(struct device *dev)
+static int __maybe_unused tegra_display_hub_suspend(struct device *dev)
 {
 	struct tegra_display_hub *hub = dev_get_drvdata(dev);
 	int err;
···
 	return 0;
 }
 
-static int tegra_display_hub_resume(struct device *dev)
+static int __maybe_unused tegra_display_hub_resume(struct device *dev)
 {
 	struct tegra_display_hub *hub = dev_get_drvdata(dev);
 	int err;