OMAPDSS: DISPC: Configure input and output sizes for writeback

Writeback uses the WB_PICTURE_SIZE register to define the size of the content
written to memory, this is the output of the scaler. It uses the WB_SIZE
register to define the size of the content coming from the overlay/manager to
which it is connected, this is the input to the scaler. This naming is different
as compared to overlays.

Add checks for writeback in dispc_ovl_set_input_size() and
dispc_ovl_set_output_size() to write to the correct registers.

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