perf, x86: Implement event scheduler helper functions

This patch introduces x86 perf scheduler code helper functions. We
need this to later add more complex functionality to support
overlapping counter constraints (next patch).

The algorithm is modified so that the range of weight values is now
generated from the constraints. There shouldn't be other functional
changes.

With the helper functions the scheduler is controlled. There are
functions to initialize, traverse the event list, find unused counters
etc. The scheduler keeps its own state.

V3:
* Added macro for_each_set_bit_cont().
* Changed functions interfaces of perf_sched_find_counter() and
  perf_sched_next_event() to use bool as return value.
* Added some comments to make code better understandable.

V4:
* Fix broken event assignment if weight of the first event is not
  wmin (perf_sched_init()).

Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1321616122-1533-2-git-send-email-robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>