drm/nouveau/svm: new ioctl to migrate process memory to GPU memory

This add an ioctl to migrate a range of process address space to the
device memory. On platform without cache coherent bus (x86, ARM, ...)
this means that CPU can not access that range directly, instead CPU
will fault which will migrate the memory back to system memory.

This is behind a staging flag so that we can evolve the API.

Signed-off-by: Jérôme Glisse <jglisse@redhat.com>