arm64: fpsimd: Simplify uses of {set,clear}_ti_thread_flag()
The existing FPSIMD context switch code contains a couple of
instances of {set,clear}_ti_thread(task_thread_info(task)). Since
there are thread flag manipulators that operate directly on
task_struct, this verbosity isn't strictly needed.
For consistency, this patch simplifies the affected calls. This
should have no impact on behaviour.