OMAP: DSS2: fix omap_dispc_register_isr() fail path

Fix handling of error in omap_dispc_register_isr() in case there are no
free isr slots available.

Reported-by: Ben Tucker <btucker@mpcdata.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/video/omap2/dss/dispc.c

@@ -2831,6 +2831,9 @@ int omap_dispc_register_isr(omap_dispc_isr_t isr, void *arg, u32 mask)
 		break;
 	}
 
+	if (ret)
+		goto err;
+
 	_omap_dispc_set_irqs();
 
 	spin_unlock_irqrestore(&dispc.irq_lock, flags);