drm/i915/pmu: Initialise our dynamic sysfs attributes for use with lockdep

As we kmalloc our dynamic sysfs attributes, we have to give them an
external static lock_class_key for them to use with lockdep.

Fixes: 109ec558 ("drm/i915/pmu: Only enumerate available counters in sysfs")
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/20180111140402.3984-1-chris@chris-wilson.co.uk