OMAPDSS: DISPC: Make dispc_ovl_setup call dispc_ovl_setup_common

Add a new static function called dispc_ovl_setup_common(). This function is used by
dispc_ovl_setup() to configure the overlay registers. This split is done so that
dispc_wb_setup() can reuse overlay register configuration related code.

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