OMAPDSS: DISPC: Add manager like functions for writeback

Add functions to enable writeback, and set/check state of GO bit. These bits are
identical in behaviour with the corresponding overlay manager bits. Configure
them in a similar way to mgr_enable() and mgr_go_* functions. Add a helper to
get the FRAMEDONE irq corresponding to writeback.

Signed-off-by: Archit Taneja <archit@ti.com>