DSP: latch dma parameters

This needs to be tested a fair bit before it could be considered a candidate for merging.
DSP wasn't latching DMA parameters appropriately and it could break the audio output.

I don't have an extensive title collection, but if people could test this PR (windows binary: https://dl.dolphin-emu.org/prs/pr-692-dolphin-latest-x64.7z) and report any regressions, that would be awesome.
(Also, some code might be polling this MMIO address for DMA status, making direct access really valuable).