[965] Improve performance by allocating CURBE buffers a page at a time.

[965] Improve performance by allocating CURBE buffers a page at a time.

Since each one is only 64b, and kernel allocations are a page anyway, this
lets us reduce buffer allocation by packing many CURBEs into one buffer, for
each batchbuffer submitted.  Improves openarena performance by around 10%.