drm/i915/pmu: Rename cpuhp_slot to cpuhp_state

Both the documentation and most of other users call the return of
cpuhp_setup_state_multi() as "state". Follow that.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241104213512.2314930-2-lucas.demarchi@intel.com


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