gallium: don't put stencil ref value in pipe_depth_stencil_alpha_state

gallium: don't put stencil ref value in pipe_depth_stencil_alpha_state

This will make driver's life a bit harder, however it makes sense that stencil
reference value is not part of the pipe_depth_stencil_alpha_state, because
it often (there are some algorithms which require this) changes more frequently
than the rest of the dsa state. This is also encouraged by some graphic APIs.
Treat it similar to pipe_blend_color.