i965: Use query->last_index instead of the global brw->query.index.

i965: Use query->last_index instead of the global brw->query.index.

Since we already have an index in the brw_query_object, there's no need
to also keep a global variable that shadows it.

Plus, if we ever add support for more types of queries that still need
the per-batch before/after treatment we do for occlusion queries, we
won't be able to use a single global variable.  In contrast, per-query
object variables will work fine.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>