mm: writeback: every 500ms the writeback daemon kicks in and flushes

the dirty pages into disk. Thats not very pleasant for performance and
*maybe* battery. This patch disables the writeback function while the
screen is on, and enables it back when the screen is off to flush all
the dirty pages. In my real world usage testing I notice a slight bump
in smoothness and performance while using the device.
Thanks to franciscoFranco