drm/amdgpu/nbio6: Correct PCIE_INDEX/DATA pair used for smn register accessing

PCIE_INDEX2/DATA2 pair will be used for smn register accessing since from vega.
PCIE_INDEX/DATA pair should be reserved for smu

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>