drm/ttm: wait for BO idle after the move in ttm_bo_swapout

Final part to avoid pre move waits.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>