ACPI: processor: Update cpuidle driver check in __acpi_processor_start()

Commit 7a8c994c ("ACPI: processor: idle: Optimize ACPI idle
driver registration") moved the ACPI idle driver registration to
acpi_processor_driver_init() and acpi_processor_power_init() does
not register an idle driver any more.

Accordingly, the cpuidle driver check in __acpi_processor_start() needs
to be updated to avoid calling acpi_processor_power_init() without a
cpuidle driver, in which case the registration of the cpuidle device
in that function would lead to a NULL pointer dereference in
__cpuidle_register_device().

Fixes: 7a8c994c ("ACPI: processor: idle: Optimize ACPI idle driver registration")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
Link: https://patch.msgid.link/5044465.31r3eYUQgx@rafael.j.wysocki


[ rjw: Changelog update ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>