drm/i915/pmu: Avoid using globals for PMU events

Attempting to bind / unbind module from devices where we have both
integrated and discreete GPU handled by i915, will cause us to try and
double free the global state, hitting null ptr deref in free_event_attributes.

Let's move it to i915_pmu.

Fixes: 05488673 ("drm/i915/pmu: Support multiple GPUs")
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200219161822.24592-2-michal.winiarski@intel.com