i965: Enumerate the pipeline statistics register counters on Gen6+.

i965: Enumerate the pipeline statistics register counters on Gen6+.

For now, we only support these on Gen6+, since that's what currently
uses hardware contexts.  When we add Ironlake hardware context support,
we can add pipeline statistics register support for that as well.

In theory, we could support pipeline statistics counters even without
hardware contexts, but it would be annoyingly painful.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>