drm/i915/pmu: Remove !CONFIG_PM code

Chris spotted that since 16ffe73c ("drm/i915/pmu: Use GT parked for
estimating RC6 while asleep") we don't rely on runtime pm internals when
estimating RC6 while asleep. We can remove the ifdef code to simplify and
at the same time wake up the device less when querying RC6 if CONFIG_PM is
not compiled in.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
References: 16ffe73c ("drm/i915/pmu: Use GT parked for estimating RC6 while asleep")
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201214094349.3563876-3-tvrtko.ursulin@linux.intel.com