tpm: crb: factor out idle/ready state transitions

Some TPM implementations have a different start method that requires
an additional notification for some state changes; for instance, the
"Pluton" start method. Just factor these transitions out for now, and
the coming commits will introduce points that the start method can hook
in at.

Reviewed by: obrien
Differential Revision: https://reviews.freebsd.org/D53682