drm/radeon: do not move bo to different placement at each cs

The bo creation placement is where the bo will be. Instead of trying
to move bo at each command stream let this work to another worker
thread that will use more advance heuristic.

agd5f: remove leftover unused variable

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>