OMAPDSS: DISPC: Configure writeback FIFOs

Extend the DISPC fifo functions to also configure the writeback FIFO thresholds.

The most optimal configuration for writeback is to push out data to the
interconnect the moment writeback pushes enough pixels in the FIFO to form a
burst. This reduces the chance of writeback overflowing it's FIFO.

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