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.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>