NVIDIA GPU: reserve load index 3 for GPU Memory, remap other loads (#2310)

* NVIDIA GPU: reserve load index 3 for GPU Memory, remap other loads

* NVIDIA GPU: simplify load sensor indexing

* NVIDIA GPU: create GPU Memory load after utilization loads

Place the GPU Memory Load sensor before power topology and D3D node loads
so it appears with other GPU metrics in the UI. Load indices advance in
the same order as before for uniqueness.

* NVIDIA GPU: use local nextLoadIndex in constructor

Replace the instance field with a ctor-local counter for Load sensor indices.