i965/gen6-7/sol: Keep independent counters for the current and previous begin/end...

i965/gen6-7/sol: Keep independent counters for the current and previous begin/end block.

This allows us to aggregate the primitive counts of a completed
transform feedback begin/end block lazily, which in the most typical
case (where glDrawTransformFeedback is not used) will allow us to
avoid aggregating the primitive counters on the CPU altogether,
preventing a stall on previous rendering during
glBeginTransformFeedback(), which dramatically improves performance of
applications that rely heavily on transform feedback.

Improves performance of SynMark2 OglGSCloth by 65.52% ±0.25% (data
gathered on VLV).

Tested-By: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>