drm/i915/pmu: Fix building without CONFIG_PM

As we peek inside struct device to query members guarded by CONFIG_PM,
so must be the code.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 1fe699e3 ("drm/i915/pmu: Fix sleep under atomic in RC6 readout")
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/20180207160428.17015-1-chris@chris-wilson.co.uk