drm/doc/rfc: Move userptr integration and vm_bind to the 'completed' section

The must-have part of the documentation was already added to the existing
/gpu/drm-vm-bind-async. The other extra discussion around GPUVM helpers
are currently active in the community. None of those discussion should
block Xe since documentation, specially around locking was completed in
a community consensus.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231201042158.80009-5-rodrigo.vivi@intel.com
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>