MIPS: Move FP usage checks into protected_{save, restore}_fp_context

In preparation for sharing protected_{save,restore}_fp_context with
compat ABIs, move the FP usage checks into said functions. This will
both enable that code to be shared, and allow for extensions of it in
further patches to also be shared.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Matthew Fortune <matthew.fortune@imgtec.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linux-kernel@vger.kernel.org
Cc: Richard Weinberger <richard@nod.at>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Maciej W. Rozycki <macro@codesourcery.com>
Patchwork: https://patchwork.linux-mips.org/patch/10790/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>