llvmpipe: Simplify vertex and geometry shaders.

llvmpipe: Simplify vertex and geometry shaders.

Eliminate lp_vertex_shader, as it added nothing over draw_vertex_shader.

Simplify lp_geometry_shader, as most of the incoming state is unneeded.
(We could also just use draw_geometry_shader if we were willing to peek
inside the structure.)

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Zack Rusin <zackr@vmware.com>