Atmosphere related scan line artifacts for Path Tracer with MGPU

There are two UDNs related to this issue.  It makes the Path Tracer unusable with MGPU, with atmosphere enabled.

I already have a one-line fix implemented and tested.  The atmosphere scan line artifacts are caused by the atmosphere lookup texture not being generated on all GPUs, so it just needs an RDG_GPU_MASK_SCOPE where that happens.

Steps reported on UDN:

1. Make sure SLI is enabled in NVIDIA Control Panel
2. Launch the project with -MaxGPUCount=2 as launch parameter
3. Create a map with PathTracingEnableReferenceAtmosphere enabled somewhere in the post process settings.
4. Switch to Path Tracing
5. Set CVAR r.PathTracing.MultiGPU 1{}

Observe scan line artifacts across the entire frame (helps to take a screenshot and zoom in).

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