drm/i915/pmu: Replace closed with registered

drm/i915/pmu: Replace closed with registered

Since i915 calls perf_pmu_register/perf_pmu_unregister, let's call the
variable "registered" so we can flip the logic and rely on it being
false by default. Looking at other drivers, it's also more common.
Examples: arch/x86/events/intel/uncore.c and
drivers/powercap/intel_rapl_common.c.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241104213512.2314930-4-lucas.demarchi@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>