drm/komeda: mark PM functions as __maybe_unused

Without this, we get a couple of warnings when CONFIG_PM
is disabled:

drivers/gpu/drm/arm/display/komeda/komeda_drv.c:156:12: error: 'komeda_rt_pm_resume' defined but not used [-Werror=unused-function]
 static int komeda_rt_pm_resume(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/arm/display/komeda/komeda_drv.c:149:12: error: 'komeda_rt_pm_suspend' defined but not used [-Werror=unused-function]
 static int komeda_rt_pm_suspend(struct device *dev)
            ^~~~~~~~~~~~~~~~~~~~

Fixes: efb46508

 ("drm/komeda: Add runtime_pm support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: james qian wang (Arm Technology China) <james.qian.wang@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200107215327.1579195-1-arnd@arndb.de