pmc_events.h: event list formatting

Improve the legibility of the list. Bump overall indentation, fix some
whitespace, and sort the IAF block.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39903

(cherry picked from commit b6c9d2a3f892169d4a6acc92869f9d9db52876ea)