drm/amdkfd: Fix SVM_ATTR_PREFERRED_LOC

The preferred location should be used as the migration destination
whenever it is accessible by the faulting GPU. System memory is always
accessible. Peer memory is accessible if it's in the same XGMI hive.

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