i965: Replace checks for rb->Name with FlipY (v2)

In the GL_MESA_framebuffer_flip_y implementation
_mesa_is_winsys_fbo checks were replaced with
FlipY checks.  rb->Name is also used to determine
if a buffer is winsys.

v2: Fixes annotation [for emil]

Fixes: ab05dd18 ("i965: implement GL_MESA_framebuffer_flip_y [v3]")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Chad Versace <chadversary@chromium.org>
(cherry picked from commit 08f97407)