ilo: use emit_SURFACE_STATE() for constant buffers

ilo: use emit_SURFACE_STATE() for constant buffers

Introduce ilo_cbuf_cso and initialize it in set_constant_buffer().  As
ilo_view_surface is embedded in ilo_cbuf_cso, switch to emit_SURFACE_STATE()
for constant buffers and remove emit_cbuf_SURFACE_STATE().