arm64: Fix finding the pmc event ID

The lower pmc event bits were masked off to find the PMC event ID.
The doesn't work when there are more events. Switch it to use the
offser relative to the first event while also checking the ID is
in the expected range.

Reviewed by: gnn, ray
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D29600