drm/amdgpu: defer test IBs on the rings at boot (V3)

amdgpu_ib_ring_tests() runs test IB's on rings at boot
contributes to ~500 ms of amdgpu driver's boot time.

This patch defers it and ensures that its executed
in amdgpu_info_ioctl() if it wasn't scheduled.

V2: Use queue_delayed_work() & flush_delayed_work().
V3: removed usage of separate wq, ensure ib tests is
    run before enabling clockgating.

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