powerpc/perf/imc: Fix nest-imc cpuhotplug callback failure

Oops is observed during boot:

  Faulting instruction address: 0xc000000000248340
  cpu 0x0: Vector: 380 (Data Access Out of Range) at [c000000ff66fb850]
      pc: c000000000248340: event_function_call+0x50/0x1f0
      lr: c00000000024878c: perf_remove_from_context+0x3c/0x100
      sp: c000000ff66fbad0
     msr: 9000000000009033
     dar: 7d20e2a6f92d03c0
    pid = 14, comm = cpuhp/0

While registering the cpuhotplug callbacks for nest-imc, if we fail in
the cpuhotplug online path for any random node in a multi node
system (because the opal call to stop nest-imc counters fails for that
node), ppc_nest_imc_cpu_offline() will get invoked for other nodes who
successfully returned from cpuhotplug online path.

This call trace is generated since in the ppc_nest_imc_cpu_offline()
path we are trying to migrate the event context, when nest-imc
counters are not even initialized.

Patch to add a check to ensure that nest-imc is registered before
migrating the event context.

Fixes: 885dcd70

 ("powerpc/perf: Add nest IMC PMU support")
Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>