arm64: Adjust the MPASS in vfp_save_state_savectx

In vfp_save_state_savectx we check if the pcb has a NULL vfp state.
When it's called multiple times with the same pcb then we can panic
because the vfp state has been set.

Weaken the requirement for the state pointer to be NULL by also
allowing it to point to the pcb vfp state area we are about to use.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47237