perf: Fix warning during perf core init

The warning was generated due to the cpu hotplug state
conflict for CPUHP_AP_PERF_ONLINE. The original idea was
to replace the perf_event_init_cpu with perf_event_start_swevents.
The fix replaces the callback in cpu.c and removes the registration
in the perf core.

Change-Id: I5cb20de38b39f3035fa98977b9c280bb20521d28
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>