hwpmc: tidy pcd_finalize methods

Make them a little bit more consistent with one another in terms of what
is done:

• Add/reduce assertions to what is most useful: a loop to ensure pcpu_fini freed everything
• Add PMCDBG trace entries

The exception is the dmc620/cmn600 classes, which behave a little
differently, so leave them untouched.

Reviewed by: jkoshy
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41270