i965: Claim to support 4 multisamples on gen6+.
i965: Claim to support 4 multisamples on gen6+.

We're not quite ready to actually support it in the implementation,
but at least this allows GL 3.0 API-reliant applications to hopefully
run successfully, though they won't get multisampling.

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

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