drm/amdgpu: use amdgpu_ring_test_helper when possible

amdgpu_ring_test_helper already handles ring->sched.ready correctly

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