hwpmc: Move 4 bits of mode to extend class size to 8

Since r289025 we have had at least 5 bits class size.
Before that it was even 16 bits, but macro handling conversion between
pmcid and set of CPU, MODE, CLASS, ROWINDEX still use 4 bits class size
and 8 bits mode size.

This breaks some libpmc API methods, like pmc_capabilities.

Since we only have 4 modes and MODE field is a number (not a bitfield)
this patch moves 4 bits of mode to extend the CLASS field.

Reviewed by: mhorne, emaste
Sponsored by: Ampere Computing LLC
Submitted by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D30047