drm/nouveau/nouveau/hmm: fix migrate zero page to GPU

When calling OpenCL clEnqueueSVMMigrateMem() on a region of memory that
is backed by pte_none() or zero pages, migrate_vma_setup() will fill the
source PFN array with an entry indicating the source page is zero.
Use this to optimize migration to device private memory by allocating
GPU memory and zero filling it instead of failing to migrate the page.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>