drm/i915/pmu: Stop setting event_init to NULL

drm/i915/pmu: Stop setting event_init to NULL

Setting event_init to NULL is mostly done to detect when the driver is
partially working: i915 probed, but pmu is not registered. However,
checking for event_init is odd as it was supposed to always be set and
kernel/events/ would just crash if it found it set to NULL.

Since there's already a "closed" boolean, use that instead and extend
it's meaning to unregistered/unregistering.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241104213512.2314930-3-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>