mm: writeback: Set the writeback daemon to screen off state
Each 1500ms the writeback daemon flushes the writeback pages into disk which causes decreasments in performance and also battery. As Powertop suggests a value of 1500ms we set this to 1500ms but with this patch it only happens when the screen is off. So the daemon can flush the all the dirty pages without taking performance from the user.  After the device wakes up and screen goes back on the writeback will be disabled.