msm: audio: qdsp6v2: Fix alternate playback issue in AC3/DTS

Buffer indexes are not getting reset at end of the playback,
hence if the file contains odd no of frames back to back
playback is not working and playback works alternatively.
In case of frames with even number of buffers these indexes
become zero when the playback completes. Fix this issue by
initializing the buffer indexes in driver open.

CRs-Fixed: 315980
Change-Id: I89cc851b7539214c8e07bb11c8a8a1b3a3158cb1
Signed-off-by: Deepa Madiregama <dmadireg@codeaurora.org>