I was thinking of the RC6 threshold parameter, but needed to consider
the RC6 evaluation interval instead. RC6 doesn't enable until activity
is below the threshold inside an evaluation interval, therefore we need
to wait at least 2 EI after idling before we can expect RC6 to be
enabled.

Fixes: 55a17bc2

 ("igt/perf_pmu: Reduce arbitrary delays before rc6")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>