i965: Fix OACONTROL assertion failures on Ironlake.

i965: Fix OACONTROL assertion failures on Ironlake.

I guarded half of the callers to start/stop_oa_counters with generation
checks, but missed the other half (which were added later).  OACONTROL
doesn't exist on Ironlake, so we better not write it.  Also, there's no
need---Ironlake's performance counters are always running.

This patch moves the generation checks into start/stop_oa_counters,
rather than requiring the caller to do them.

Fixes assertion failures in Piglit's AMD_performance_monitor/measure.

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