i965: Don't bother trying to extend the current vertex buffers.

i965: Don't bother trying to extend the current vertex buffers.

This essentially reverts the following:

  commit c625aa19cb53ed27f91bfd16fea6ea727e9a5bbd
  Author: Chris Wilson <chris@chris-wilson.co.uk>
  Date:   Fri Feb 18 10:37:43 2011 +0000

      intel: extend current vertex buffers

While working on optimizing an upcoming Steam title, I broke this code.
Eric expressed his doubts about this optimization, and noted that the
original commit offered no performance data.

I ran before and after benchmarks on Xonotic and Citybench, and found
that this code made no difference.  So, remove it to reduce complexity
and make future work simpler.

Reviewed-by: Eric Anholt <eric@anholt.net>