drm/i915/pmu: Use event_to_pmu()

i915 pointer is not needed in this function and all the others simply
calculate the i915_pmu container based on the event->pmu. Follow the
same logic as in other functions.

Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240909204340.3646458-2-lucas.demarchi@intel.com


Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>