powerpc/perf: IMC code cleanup with some code refactoring

Factor out memory freeing part for attribute elements from
imc_common_cpuhp_mem_free().

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>