amdgpu/dc: make some audio functions return void

There is no need to check for these pointers being valid
at this level. Check earlier if required.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>