drm/hisilicon/hibmc: mark PM functions __maybe_unused

When CONFIG_PM_SLEEP is disabled, we get a harmless warning

drm/hisilicon/hibmc/hibmc_drm_drv.c:115:12: error: ‘hibmc_pm_resume’ defined but not used [-Werror=unused-function]
drm/hisilicon/hibmc/hibmc_drm_drv.c:97:12: error: ‘hibmc_pm_suspend’ defined but not used [-Werror=unused-function]

Marking the functions as __maybe_unused avoids the warning without
having to add an #ifdef.

Fixes: 5e0df3a08f3d ("drm/hisilicon/hibmc: Add hisilicon hibmc drm master driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161124163107.3914495-1-arnd@arndb.de

drivers/gpu/drm/hisilicon/hibmc/hibmc_drm_drv.c

@@ -94,7 +94,7 @@ static struct drm_driver hibmc_driver = {
 	.irq_handler		= hibmc_drm_interrupt,
 };
 
-static int hibmc_pm_suspend(struct device *dev)
+static int __maybe_unused hibmc_pm_suspend(struct device *dev)
 {
 	struct pci_dev *pdev = to_pci_dev(dev);
 	struct drm_device *drm_dev = pci_get_drvdata(pdev);
@@ -112,7 +112,7 @@ static int hibmc_pm_suspend(struct device *dev)
 	return 0;
 }
 
-static int hibmc_pm_resume(struct device *dev)
+static int  __maybe_unused hibmc_pm_resume(struct device *dev)
 {
 	struct pci_dev *pdev = to_pci_dev(dev);
 	struct drm_device *drm_dev = pci_get_drvdata(pdev);