perf_counter, x86: change and remove pmu initialization checks

Some functions are only called if the pmu was proper initialized. That
initalization checks can be removed. The way to check initialization
changed too. Now, the pointer to the interrupt handler is checked. If
it exists the pmu is initialized. This also removes a static variable
and uses struct x86_pmu as only data source for the check.

[ Impact: simplify code ]

Signed-off-by: Robert Richter <robert.richter@amd.com>
Cc: Paul Mackerras <paulus@samba.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1241002046-8832-24-git-send-email-robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>