igt/perf_pmu: Measure the reference batch for busy-check-all

Don't rely on the timer being precise when we can sleep for a known
duration.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
1 file changed