Refactor MMC3 IRQs to be actually based on the A12 line

[rynorris]

Right now my MMC3 implementation only finds the value of the A12 line whenever the PPU reads from the mapper.
Technically it should just know the actual state of A12 at all times, otherwise the counter won't decrement because of writes to PPUADDR.

I need to refactor slightly so that the MMC3 can have direct access to the value of ppu.v somehow.