i965/wm: emit null buffer surfaces when null buffers are attached

i965/wm: emit null buffer surfaces when null buffers are attached

Otherwise we can expect odd things to happen if, for example, we ask
for the size of the attached buffer from shader code, since that
might query this value from the surface we uploaded and get random
results.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>