gallium: overhaul usage of vertex_info in draw module.

gallium: overhaul usage of vertex_info in draw module.

Remove all dependencies on vertex_info, except for draw_vbuf.
Drawing stages now strictly operate on post-transformed vertices and don't
know anything about hw vertices.
Use vertex program output info for two-side/flat/etc stages.
Temporarily disable vbuf module in softpipe driver.