OMAPDSS: move irq handling to dispc-compat

The whole dispc irq handling system we currently have is only needed for
compat layer, and thus can be moved from dispc.c to the compat layer.

This is quite straigtforward, but we need to add new dispc functions to
request and free the actual hardware irq: dispc_request_irq() and
dispc_free_irq().

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>