drm/amdgpu:fix cs_ib_fill

should use chunk_ib instead of ib, otherwise the logic
is incorrect.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Ken Wang <Qingqing.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>