hwpmc: pass pmc pointer to more class methods

[mhorne]

Summary

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.

[mhorne]

Tinderbox points out that I didn't finish the change for a few platforms.

[mhorne]

Update other classes; it now passes tinderbox.

[jkoshy]

FYI, the pm_id field in a struct pmc contains the CPU and 'row index' for the PMC, so we could potentially drop the cpu and ri fields in a future change.