Rather than create the pixmap, this uses the file descriptor
to change an existing pixmaps backing store.
This is required for reverse prime slaves, where we create
the slave pixmap, then set the backing store.
v1.1: use local pScreen (Eric)
Reviewed-by: Eric Anholt <eric@annholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>