drm/doc/rfc: Mark GPU VA as complete.

Nouveau has landed the GPU VA helpers, support and documentation
already and Xe is already using the upstream GPU VA.

Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/commit/ea4ae69e66b2940107e74f240ecb9dae87bf1ff1
Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/commits/drm-xe-next?ref_type=heads


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/20230829163005.54067-4-rodrigo.vivi@intel.com