rbug: unwrap index buffer resource

All resources passed to the drivers below rbug need to be unwrapped before
being passed down. We missed to do this for the index buffer resource when
this was made part of the draw_info structure.

Fixes: 330d0607 (gallium: remove pipe_index_buffer and set_index_buffer)
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
(cherry picked from commit a75eb888)