MIPS: Factor out FPU feature probing

Factor out FPU feature probing, mainly to remove code duplication from
`fpu_disable'.  No functional change although shuffle some code to avoid
forward references.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9712/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>