drm/amdgpu: Add userptr support for KFD

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

v2: updated a comment

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>