drm/amdkfd: Remove prefault before migrating to VRAM

Prefaulting potentially allocates system memory pages before a
migration. This adds unnecessary overhead. Instead we can skip
unallocated pages in the migration and just point migrate->dst to a
0-initialized VRAM page directly. Then the VRAM page will be inserted
to the PTE. A subsequent CPU page fault will migrate the page back to
system memory.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>