drm/doc/rfc: Move Xe 'ASYNC VM_BIND' to the 'completed' section

As already indicated in this block, the consensus was already
reached out and documented as:
The ASYNC VM_BIND document </gpu/drm-vm-bind-async>

However this was item was not moved to the completed section.
Let's move and clean up the WIP block.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231201042158.80009-4-rodrigo.vivi@intel.com
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>