drm/i915/pmu: Cheat when reading the actual frequency to avoid fw

We want to avoid taking forcewake when querying the performance stats,
as we wish to avoid perturbing the system under observation. (And with
the forcewake being kept alive for 1ms after use, sampling the frequency
from a 200Hz timer keeps forcewake 40% active.)

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>
Link: https://patchwork.freedesktop.org/patch/msgid/20191108103511.20951-1-chris@chris-wilson.co.uk