intel: add comment explaining weird-looking code

Merged Eric Engestrom requested to merge eric/mesa:intel-var-sized-type-explanation into master

Clang throws warnings here (-Wgnu-variable-sized-type-not-at-end) because of a variable-sized array at the end of drm_i915_query_topology_info being aliased by the data field in topology, but that's the point here, so let's explain it so that the next me doesn't blindly listen to the compiler :)

Suggested-by: @brianp
/cc @tpalli @llandwerlin

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