Revert "PM QoS: Use spinlock in the per-device PM QoS constraints code"

This reverts commit fc2fb3a0.

The problem with the above commit is that it makes the device PM QoS
core code hold a spinlock around blocking_notifier_call_chain()
invocations.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>