libweston: Move plane stack/release for output primary_plane

It doesn't make sense to stack the plane before it's useful - so only
put it in the compositor's plane list on output_enable. The opposite of
weston_output_enable is weston_compositor_remove_output, so release the
plane there.

This stops a crash when closing one of multiple windows for a nested
backend results in the output being freed while the plane is still on the
compositor's plane list.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>