OMAPDSS: DISPC: Configure writeback specific parameters in dispc_wb_setup()

Configure some of the writeback specific parameters in dispc_wb_setup(). The
writeback parameters configured are:

truncation: This needs to be set if the color depth input to writeback is more
than the color depth of the color mode we want to store in memory.

writeback mode: This configures whether we want to use writeback in mem to mem
or capture mode. This information will be directly passed by APPLY later.

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