i965: Remove nr_surfaces computation from brw_update_wm_surfaces.

i965: Remove nr_surfaces computation from brw_update_wm_surfaces.

This code is fairly fragile, as it depends on the ordering of the
entries in the binding table, which will change soon.

Also, stop listening on the BRW_NEW_WM_CONSTBUF dirty bit as it's no
longer required.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>