amdgpu/dc: another round of dce/dcn construct cleanups.

This removes any remaining pointless return codepaths from the
DCE code.

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>