Take request_channel_layout as a hint and don't force 2.0 downmix by
using both the 2CH and 6CH flags together.

Remove warnings about missing coefficients because they are no longer
relevant.

Honor AV_CH_LAYOUT_NATIVE and make it possible for native DTS channel
layout to be output.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>