xdg-shell: Move check for surface having buffer earlier

It's illegal to create an xdg_surface for a surface which already has a
buffer attached to it. We check for this, but only after we've created
our weston_desktop_surface; this simply avoids creating the internal
tracking structure when we're only going to destroy it after posting the
error.

Signed-off-by: Daniel Stone <daniels@collabora.com>