Accurate loading times for Wii games

This is a continuation of [PR 1634](https://github.com/dolphin-emu/dolphin/pull/1617). WII_IPC_HLE_Device_DI now uses all the timing accuracy improvements that DVDInterface has gotten throughout this year. Also, the fast disc speed option now works for Wii games. The second and third commits might cause problems in games that rely on specific IPC timings, so testing is needed.
Is "+=" actually right here, as opposed to std::max?  I actually don't know the answer here.