Only force iLoopFilterDisableIdc to 2 if multithreading has been enabled

Previously these lines were within an #ifdef MT_ENABLED block,
but now that threading is enabled by default, we should probably
only do it if threading has been requested.
Merge pull request #1060 from mstorsjo/loopfilter-multithreading

Only force iLoopFilterDisableIdc to 2 if multithreading has been enabled