gallium: Implement draw_vbo and set_index_buffer for all drivers.

gallium: Implement draw_vbo and set_index_buffer for all drivers.

Some drivers define a generic function that is called by all drawing
functions.  To implement draw_vbo for such drivers, either draw_vbo
calls the generic function or the prototype of the generic function is
changed to match draw_vbo.

Other drivers have no such generic function.  draw_vbo is implemented by
calling either draw_arrays and draw_elements.

For most drivers, set_index_buffer does not mark the state dirty for
tracking.  Instead, the index buffer state is emitted whenever draw_vbo
is called, just like the case with draw_elements.  It surely can be
improved.