FIFO is overflown by GatherPipe, CPU thread is too fast! (4.0-926)

[tueidj]

Obviously I don't know that part of Dolphin's code too well but the hardware has safeguards to prevent exactly this issue (the high and low watermarks) so it definitely seems like a problem with the implementation (and not something that should be caused by having separate CPU and GPU threads; the CPU only does writes and the GPU only does reads). Does bursting the gather pipe not always trigger an exception check (at least when the CP<->GP fifo link is active)?