hwpstate_amd(4): Factor out setting the CPPC_REQUEST register

In preparation for creating other knobs to tweak values in this register
beyond just the EPP (Efficiency/Performance Preference).

While here, add a herald comment before the softc structure indicating
how we achieve atomicity when modifying the softc.

Reviewed by: aokblast
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55008