drm/doc/rfc: Mark DRM_VM_BIND as complete.

The consensus is for individual drivers VM_BIND uapis with
the GPUVA helpers that are already implemented and merged
upstream.

The merged GPUVA documentation also establish some overall
rules for the locking to be followed by the drivers.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230829163005.54067-3-rodrigo.vivi@intel.com