swr: [rasterizer core] split FE and BE stats

swr: [rasterizer core] split FE and BE stats

Separated FE stats out into its own structure.  There are 17 FE vs 3 BE
stat fields.  Since there is only one FE thread per DC then we don't have
to loop over all threads and sum up FE stats over all the worker threads.
This also reduces size of DC since we only need to store one copy of the
FE stats and not one per worker.  Finally, we can use the new FE callback
mechanism to update these.

Signed-off-by: Tim Rowley <timothy.o.rowley@intel.com>