llvmpipe: reorganization of binning data structions and funtions

llvmpipe: reorganization of binning data structions and funtions

New lp_bins struct contains all bin information.
More move bin-related code into lp_bin.[ch]
Use new/updated bin-access functions to hide implementation details.
The result is more/cleaner separation between the setup and rast components.
This will make double-buffering of the bins easier, etc.