Correct some basic mpeg functionality to match tests

This improves the tests in hrydgard/pspautotests#128, although the sceMpegRingbufferAvailableSize() one doesn't work yet.  It looks like "packetsFree" is wrong and it's actually "packetsUsed", or something, but I probably need to do more testing.

Unlikely to fix anything, though.

-[Unknown]