Perf: arm64: enable idle_update at end of probe

Currently armv8pmu_idle_update is enabled before
arm_pmu_device_probe has completed. arm_pmu_device_probe
initializes some structures need by armv8pmu_idle_update.
armv8pmu_idle_update can get called before these structures
are complete leading to seg faults. Set a flag to prevent
armv8pmu_idle_update from trying to access these structures
until arm_pmu_device_probe is finished. Also move the call
to perf_event_cpu_hp_init after arm_pmu_device_probe so
that hotplug doesn't overwrite the flag.

Change-Id: I695eaa396d4df23dcee72dda41f342d865a27d08
Signed-off-by: Patrick Fay <pfay@codeaurora.org>