oprofile/x86: fix perfctr nmi reservation for mulitplexing

Multiple virtual counters share one physical counter. The reservation
of virtual counters fails due to duplicate allocation of the same
counter. The counters are already reserved. Thus, virtual counter
reservation may removed at all. This also makes the code easier.

Cc: stable@kernel.org
Signed-off-by: Robert Richter <robert.richter@amd.com>