intel_stub: Report something for I915_CONTEXT_PARAM_GTT_SIZE

Kenneth Graunke requested to merge kwg/shader-db:stub-gtt into master

iris won't load without this returning a value larger than 4GB. i965 will use softpin on Gen8+ if it sees a value larger than 4GB.

This is technically wrong for Gen7.5 and older, but i965 doesn't use the value for anything in that case so it doesn't really matter.

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