gallium: add void *user_buffer in pipe_vertex_buffer

gallium: add void *user_buffer in pipe_vertex_buffer

This reduces CPU overhead in st_draw_vbo and removes a lot of unnecessary code
in that function which was required only to comply with the gallium interface,
but wasn't any useful really.

Adapted drivers: i915, llvmpipe, r300, softpipe.
No changes required in: r600, radeonsi.

User vertex buffers have been disabled in nv30, nv50, nvc0 and svga to keep
things working.