powerpc/pseries: convert CMO probe to papr_sysparm API

Convert the direct invocation of the ibm,get-system-parameter RTAS
function to papr_sysparm_get().

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230125-b4-powerpc-rtas-queue-v3-15-26929c8cce78@linux.ibm.com