wined3d: Also load the index buffer in wined3d_context_vk_apply_draw_state() if STATE_INDEXBUFFER was invalidated.

Otherwise we might try to bind a NULL buffer if the index buffer was not
previously loaded.
Signed-off-by: Henri Verbeet <hverbeet@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>