audio: apply volume on DirectOutput streams

- Call stream->set_volume to apply volume on
  particular stream of DirectOutputs

Id: I98e3b856ee508d407f893afad66caade5eda3e4a
CRs-Fixed: 445953
http://gerrit.sudoservers.com/#/c/6718/

Change-Id: I1e0b4d08a6176caf3da6951b5610e95297b09289
Signed-off-by: DHO <DHO@dho.im>