perf/x86/amd/uncore: Plug reference leak

In the error path of amd_uncore_cpu_up_prepare() the newly allocated uncore
struct is freed, but the percpu pointer still references it. Set it to NULL.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1602162302170.19512@nanos


Signed-off-by: Ingo Molnar <mingo@kernel.org>