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: 5e0df3a0 ("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