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
(cherry picked from commit f07ef2d9c9e9d1e84c532e9102594834e88a5c83)

Conflicts:

	libavresample/audio_mix.c
Prepare for 9.2 Release