x86: initialize initial FPU state earlier

Make it under SI_SUB_CPU sysinit, instead of much later SI_SUB_DRIVERS.
The SI_SUB_DRIVERS survived from times when FPU used real ISA attachment,
now it is only pnp stub claiming id.

PR: 255997
Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30512