drm/nouveau/volt: use kernel's 64-bit signed division function

Doing direct 64 bit divisions in kernel code leads to references to
undefined symbols on 32 bit architectures. Replace such divisions with
calls to div64_s64 to make the module usable on 32 bit archs.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>