adding handles to pm_qos?

["Ai Li" <aili@codeaurora.org>]

We are calling pm_qos from some of our drivers.  One concern is that
each call of pm_qos_update_requirement() iterates through the client
list and strcmp the client names.  It could be slow.  A proposal is
that pm_qos provides handles that can be used on
pm_qos_update_requirement().

For measurement purposes, I added get/put interfaces to
acquire/release the handles and a new pm_qos_update_requirement
function that bypasses the iteration and strcmp.  Here are some
collected data:

How many clock cycles does pm_qos_update_requirement take?
when there is one client on this qos_class:
    using handle        using name      using handle/using name
avg 252.4               400.4           63%

when there are 5 clients on this qos_class:
    using handle        using name      using handle/using name
avg 407.6               644.8           63%

when there are 10 clients on this qos_class:
    using handle        using name      using handle/using name
avg 582.4               938.4           62%

Given the time differences, it seems worthwhile to add handles.

~Ai