mca: Handle inconsistent CMCI capability reporting

A BIOS bug may apparently cause the BSP to report that it does not
implement CMCI, with some APs reporting that they do. In this scenario,
avoid a NULL pointer dereference that occurs in cmci_monitor() because
cmc_state was not allocated by the BSP.

PR: 253272
Reported by: asomers, mmacy
Reviewed by: kib (previous version)

(cherry picked from commit b5770470276268acef21368b3e77a325df883500)