drm/ttm: handle the SYSTEM->TT path in same place as others.

This just consolidates the code making the flow easier to understand
and also helps when moving move to the driver side.

Reviewed-by: Christian König <christian.koenig@amd.com&gt;.>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924051845.397177-3-airlied@gmail.com