hwpstate_amd(4): Register dump: Fine-grained error reporting

If some of the registers cannot be read, report that but continue trying
reading the others. This also has the side benefit of simplifying code.

While here, use sbuf_new_for_sysctl(), and rename 'res' and 'ret', which
are to contain error values, to 'error'.

While here, remove the test on getting the per-cpu structure, as if it
is not present we would have already crashed on device attach.

While here, fix format for printing the CPU ID.

PR: 292615
Reviewed by: aokblast (older version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55005