drm/ttm: call ttm_bo_swapout directly when ttm shrink

remove the extra indirection because we have only one implementation anyway

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