i965: Don't create constant buffers if they won't be used.
i965: Don't create constant buffers if they won't be used.

Really, the creation and upload of constants should be in the same place,
since they should only happen together, and a state flag should be
triggered by them so that we don't thrash state around so much for just
updating constants.  But this still recovers openarena performance by
another 19%, leaving us 16% behind Mesa 7.4 branch.

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