powerpc/perf/hv-24x7: Define add_event_to_24x7_request()

Move code that maps a perf_event to a 24x7 request buffer into a
separate function, add_event_to_24x7_request().

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>