gallium: modify draw_find_vs_output() to search vertex shader outputs

gallium: modify draw_find_vs_output() to search vertex shader outputs

This simplifies drivers using the draw module and removes the last dependency
on vertex-shader "internals".  Since the draw module is producing the
post-transformed vertices, it makes sense to ask it where specific vertex
attributes are located.

This could also simplify some things in the state tracker code for selection,
feedback, rasterpos...