drm/omap: dispc: disp_wb_setup to check return code

When dispc_wb_setup() calls dispc_ovl_setup_common() it does not
check for failure but instead keeps on partially setting up WB.
This causes the WB H/W to be partially initialized and yield
unexpected behavior.

Make sure return code is successful before proceeding.

Signed-off-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>