drm/amdgpu/ttm: drop the adev link from vram mgr

There is no need for that now since it's embedded.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200806233459.4057784-3-airlied@gmail.com