perf: Fix perf_init_event()

We ought to return -ENOENT when non of the registered PMUs
recognise the requested event.

This fixes a boot crash that occurs if no PMU is available
but the NMI watchdog tries to register an event.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>