drm/amdgpu: cleanup si_dma_ring_test_ib

Accidentially missed during the last cleanup.

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