post_proc: fix noise at start of multichannel playback in hw_acc mode

BufferProvider in hw_accelerator module is intialized with Downmix
bufferProvider instead of track inputBufferProvider if Downmix module
is configured in AudioMixer, this results in improper mixbuffer data
for first threadloop_mix.
Hw_accelerator module is updated with track inputBufferProvider
correctly in next threadloop cycle when setBufferProvider is called
again, resulting in correct output for rest of the playback.
Fix this issue by using track inputBufferProvider as bufferProvider
of hw_accelerator module during initial configuration.

CRs-Fixed: 868620
Change-Id: Ia8b8ef472ff881d2de3b8094a8a48ce63a1a40cb