pipe-loader: rework the sw backend

pipe-loader: rework the sw backend

Move the winsys into the pipe-target, similar to the hardware
pipe-driver.

v2:
 - move int declaration outside of loop (Brian)
 - fold the teardown into a goto + separate function.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Rob Clark <robclark@freedesktop.org>