thus amdgpu_ib_free() can hook sched fence to SA manager
in later patches.

BTW:
for amdgpu_free_job(), it should only fence_put() the
fence of the last ib once, so fix it as well in this patch.
Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>