drm/omap: add define for DISPC_IRQ_WBUNCOMPLETEERROR

OMAP4+ DSS has WBUNCOMPLETEERROR irq, which was not defined in the irq
list. Add the define.

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