This commit is mostly just a cosmetic change that cleans-up the interfaces,
replacing pipe_winsys::surface_* calls by

   /**
    * Allocate storage for a display target surface.
    *
    * Often surfaces which are meant to be blitted to the front screen (i.e.,
    * display targets) must be allocated with special characteristics, memory
    * pools, or obtained directly from the windowing system.
    *
    * This callback is invoked by the pipe_screenwhen creating a texture marked
    * with the PIPE_TEXTURE_USAGE_DISPLAY_TARGET flag  to get the underlying
    * buffer storage.
    */
   struct pipe_buffer *(*surface_buffer_create)(struct pipe_winsys *ws,
                                        unsigned width, unsigned height,
                                        enum pipe_format format,
                                        unsigned usage,
                                        unsigned *stride);

Most drivers were updated but not all were tested. Use the softpipe pipe
driver and the xlib winsys changes as a reference when fixing other drivers.