r600g: Add r600_context_pipe_state_emit()

r600g: Add r600_context_pipe_state_emit()

This function is used when dispatching compute shader in order to avoid
mixing compute and 3D registers in the context's dirty list.  This
allows the compute code to resuse 3D functions like evergreen_cb, which
return a struct r600_pipe_state and still have control over when and how
the register writes are emitted.