drm/amdkfd: handle errors returned by svm_migrate_copy_to_vram/ram

If migration copy failed because process is killed, or out of VRAM or
system memory, pass error code back to caller to handle error
gracefully.

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>