lavr: call mix_function_init() in ff_audio_mix_set_matrix()

This is needed if a custom matrix is set by the user after opening the
AVAudioResampleContext because the matrix channel count can change if
different mixing coefficients are used.

CC:libav-stable@libav.org
lavr: always reset mix function names and pointers in mix_function_init()

CC: libav-stable@libav.org