intel: Make more consistent use of _mesa_is_{user,winsys}_fbo()

intel: Make more consistent use of _mesa_is_{user,winsys}_fbo()

A lot of code was still differentiating between between winsys and
user fbos by testing the fbo's name against zero.  This converts
everything in the i915 and 965 drivers over to use _mesa_is_user_fbo()
and _mesa_is_winsys_fbo().

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>