i965/gen6-7/sol: Restructure primitive counter into a separate type.

i965/gen6-7/sol: Restructure primitive counter into a separate type.

A primitive counter encapsulates a scalar aggregating counter for each
vertex stream along with a section within the primitive tally buffer
which hasn't been read out yet.  Defining this as a separate type will
allow us to keep multiple counter objects around for the same
transform feedback object without any code duplication.

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