This is set by pre_init not screen init, so if we free it here
and then recycle the server, we lose all the providers.
I think we need to wrap FreeScreen here to do this properly,
will investigate for 1.14 most likely, safer to just leak this
on server exit for now.
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>