hal: expose offload functions only for offload playback

- Expose compress offload related functions only when
  AUDIO_OUTPUT_FLAG_COMPRESS_OFFLOAD is set.
- Do not use them for direct PCM format to avoid unnecessary
  calls to compress driver.

CRs-Fixed: 913925
Change-Id: I8f93b1c697794e686d19c6c78823a095333b5369