iommu/vt-d: Clean up cpumask and hotplug for perfmon

The iommu PMU is system-wide scope, which is supported by the generic
perf_event subsystem now.

Set the scope for the iommu PMU and remove all the cpumask and hotplug
codes.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20240802151643.1691631-5-kan.liang@linux.intel.com