powerpc/perf: Fix crash in perf_instruction_pointer() when ppmu is not set

      On systems without any specific PMU driver support registered, running
perf record causes Oops.

The relevant portion from call trace:

  BUG: Kernel NULL pointer dereference on read at 0x00000040
  Faulting instruction address: 0xc0021f0c
  Oops: Kernel access of bad area, sig: 11 [#1]
  BE PAGE_SIZE=4K PREEMPT CMPCPRO
  SAF3000 DIE NOTIFICATION
  CPU: 0 PID: 442 Comm: null_syscall Not tainted 5.13.0-rc6-s3k-dev-01645-g7649ee3d2957 #5164
  NIP:  c0021f0c LR: c00e8ad8 CTR: c00d8a5c
  NIP perf_instruction_pointer+0x10/0x60
  LR  perf_prepare_sample+0x344/0x674
  Call Trace:
    perf_prepare_sample+0x7c/0x674 (unreliable)
    perf_event_output_forward+0x3c/0x94
    __perf_event_overflow+0x74/0x14c
    perf_swevent_hrtimer+0xf8/0x170
    __hrtimer_run_queues.constprop.0+0x160/0x318
    hrtimer_interrupt+0x148/0x3b0
    timer_interrupt+0xc4/0x22c
    Decrementer_virt+0xb8/0xbc

During perf record session, perf_instr