drm/amdgpu: fix s4 resume

No need to re-init asic if it's already been initialized.
Skip IB tests since kernel processes are frozen in thaw.

Signed-off-by: Flora Cui <Flora.Cui@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org