perf, x86: P4 PMU - Fix unflagged overflows handling

Don found that P4 PMU reads CCCR register instead of counter
itself (in attempt to catch unflagged event) this makes P4
NMI handler to consume all NMIs it observes. So the other
NMI users such as kgdb simply have no chance to get NMI
on their hands.

Side note: at moment there is no way to run nmi-watchdog
together with perf tool. This is because both 'perf top' and
nmi-watchdog use same event. So while nmi-watchdog reserves
one event/counter for own needs there is no room for perf tool
left (there is a way to disable nmi-watchdog on boot of course).

Ming has tested this patch with the following results

 | 1. watchdog disabled
 |
 | kgdb tests on boot OK
 | perf works OK
 |
 | 2. watchdog enabled, without patch perf-x86-p4-nmi-4
 |
 | kgdb tests on boot hang
 |
 | 3. watchdog enabled, without patch perf-x86-p4-nmi-4 and do not run kgdb
 | tests on boot
 |
 | "perf top" partialy works
 |   cpu-cycles            no
 |   instructions          yes
 |   ca...