drm/amdgpu: make IB test synchronize with init for SRIOV(v2)

issue:
originally we kickoff IB test asynchronously with driver's
init, thus
the IB test may still running when the driver loading
done (modprobe amdgpu done).
if we shutdown VM immediately after amdgpu driver
loaded then GPU may
hang because the IB test is still running

fix:
flush the delayed_init routine at the bottom of device_init
to avoid driver loading done prior to the IB test completes

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