i965: Write code to compute a VUE map.

i965: Write code to compute a VUE map.

Several places in the i965 code make implicit assumptions about the
structure of data in the VUE (vertex URB entry).  This patch adds a
function, brw_compute_vue_map(), which computes the structure of the
VUE explicitly.  Future patches will modify the rest of the driver to
use the explicitly computed map rather than rely on implicit
assumptions about it.

Reviewed-by: Eric Anholt <eric@anholt.net>