sink-input, source-output: Assign to volume from only one place

sink-input, source-output: Assign to volume from only one place

Forcing all volume changes to go through set_volume_direct() makes
it easier to check where the stream volume is changed, and it also
allows us to have only one place where notifications for changed
volume are sent.

Change-Id: Ie61bcc5747b419bb83c19a3ed78fd9f4d8a73cce
Signed-off-by: Jaska Uimonen <jaska.uimonen@intel.com>