drm/amdgpu/vcn: use amdgpu_ring_test_helper

Instead of amdgpu_ring_test_ring, so the helper function determines
whether the ring is ready

Signed-off-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: James Zhu <James.Zhu@amd.com>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>