hwpmc: pass pmc pointer to more class methods

In many cases this avoids an extra lookup, since the callers always have
pm at hand. We can also eliminate several assertions, mostly for pm !=
NULL. The class methods are an internal interface, and the callers
already handle such a scenario. No functional change intended.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39915