Is it mandatory to have a dedicated link (trunk)as failover/state failover link between the two switches for an FWSM Interchassis failover?
It's not mandatory to have a "dedicated link" for failover no but it is recommended practice. You can use an existing trunk link that carries other vlan traffic.
The suggestion to use a dedicated link is to ensure that the link does not get flooded with normal data traffic which could lead to problems with the failover.
It all depends on how busy your existing layer2 trunk links are.