This adds support for allocating, mapping, unmapping and freeing
userptr BOs, and for handling MMU notifiers.

v2: updated a comment
Signed-off-by: NFelix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: NOded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: NOded Gabbay <oded.gabbay@gmail.com>