GPU: Fire GPU interrupts at the correct places.

PDC0 and PDC1 are both VBlank interrupts. PDC0 was being treated as a
HBlank interrupt and fired many more times than it should. They now both
fire together at 60 Hz. This puzzlingly *improves* apparent framerate on
many applications.

A few other interrupts were being fired inside the GSP command
processing instead of on the actual GPU register writes, so they were
moved there, which should cover direct writes tho those registers not
going through the GX command queue.