amdgpu: fix NULL pointer dereference at tonga_check_states_equal

The event_data passed from pem_fini was not cleared upon initialization.
This caused NULL checks to pass and cast_const_phw_tonga_power_state to
attempt to dereference an invalid pointer. Clear the event_data in
pem_init and pem_fini before calling pem_handle_event.

Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Bradley Pankow <btpankow@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>