[Open-graphics] Video interrupt enable/disable

[Paul Brook]

> > IMHO the sync interrupt doesn't need to be different from to any other
> > interrupt.
>
> If the interrupt is the same for sync and service request, then the
> driver will have to read two status register bits to see which interrupt
> is set before the interrupt is serviced and then write back to clear the
> interrupt.

Chances are you're going to have to do that anyway because you might be on a 
shared interrupt line.

Paul