iris: Properly mark vertex buffers as unbound
Commit 0278d1fa added a new parameter to pipe->set_vertex_buffers() which specifies a number of trailing slots to unbind. iris_set_vertex_buffers wasn't taking this into account, so it was failing to mark those as unbound. A simple addition is all that's needed.
Fixes: 0278d1fa ("gallium: add unbind_num_trailing_slots to set_vertex_buffers")