pipe-loader: Don't destroy the winsys in the sw loader

pipe-loader: Don't destroy the winsys in the sw loader

The screen takes ownership of the winsys, and is responsible for
destroying it.  Users of pipe-loader should make sure they destory
and  screens they've created to avoid memory leaks.

This fixes a crash in clover introduced by
ce6c17c0833032e91a2d1b34f9eb80c738a854a2 where the pipe-loader was
destroying the winsys while a screen was still using it.

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>