drm/amdgpu/nbio7.4: add hw bug workaround for vega20

Configure PCIE_CI_CNTL to work around a hw bug that affects
some multi-GPU compute workloads.

Acked-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Harish Kasiviswanathan <Harish.Kasiviswanathan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>