[omxplayer] Make use of TrueHD fastpath when downmixing

The TrueHD codec actually works in 3 stages.
It decodes the downmixed stereo.
It then decodes the differences required to produce 5.1.
It then decodes the differences required to produce 7.1.

Many users end up downmixing this 7.1 stream back to 2.0.
Much better to tell the codec we only need the 2.0 stream.
It saves about 50% of the CPU required