When providers get blocked unregister_dca_providers() is called ending up
with dca_providers and dca_domain lists emptied. Dca should be prevented from
trying to unregister any provider if dca_domain list is found empty.

Cc: <stable@vger.kernel.org>
Reported-by: NJiang Liu <jiang.liu@huawei.com>
Tested-by: NGaohuai Han <hangaohuai@huawei.com>
Signed-off-by: NMaciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: NDan Williams <djbw@fb.com>