i965/fs: Don't emit a header on gen5+ sample messages unless required.
i965/fs: Don't emit a header on gen5+ sample messages unless required.

Improves glbenchmark egypt performance 0.6% +/- 0.4% (n=6).

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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