omap: mailbox: call request_irq after mbox queues are allocated

The OMAP mailbox startup code is enabling the interrupt before any
of the associated mailbox queues are allocated. Move this code so
that the interrupt configuration for a mailbox is together.

Signed-off-by: Fernando Guzman Lugo <lugo.fernando@gmail.com>
Signed-off-by: Suman Anna <s-anna@ti.com>