intel: Move intel_draw_buffers() code into each driver.

The illusion of shared code here wasn't fooling anybody.  It was
tempting to keep i830 and i915 still shared, but I think I actually
want to make them diverge shortly.

Reviewed-by: Chad Versace <chad@chad-versace.us>

src/mesa/drivers/dri/intel/intel_context.h

@@ -136,6 +136,7 @@ struct intel_context
                                struct intel_region * draw_regions[],
                                struct intel_region * depth_region,
 			       GLuint num_regions);
+      void (*update_draw_buffer)(struct intel_context *intel);
 
       void (*reduced_primitive_state) (struct intel_context * intel,
                                        GLenum rprim);