drm/vmwgfx/ttm: fix the non-THP cleanup path.

I fixed the init path, but missed the cleanup path.

Fixes: e0830704 ("drm/vmwgfx: takedown vram manager")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200810054110.4192239-1-airlied@gmail.com