drivers/perf: arm_pmu: Defer the setting of __oprofile_cpu_pmu

The global variable __oprofile_cpu_pmu is set before the PMU is fully
initialized. If an error occurs before the end of the initialization,
the PMU will be freed and the variable will contain an invalid pointer.

This will result in a kernel crash when perf will be used.

Fix it by moving the setting of __oprofile_cpu_pmu when the PMU is fully
initialized (i.e when it is no longer possible to fail).

Cc: <stable@vger.kernel.org>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>