perf diff: Move diff related columns into diff command

Moving diff related columns into diff command, because they are not used
by any other command.

Also moving the column entry functions under generic one with baseline
as an exception.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-v58qfl75xkqojz54h1v5fy6p@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>