perf tools: Use an accessor to read thread comm

As the thread comm is going to be implemented by way of a more
complicated data structure than just a pointer to a string from the
thread struct, convert the readers of comm to use an accessor instead of
accessing it directly.

The accessor will be later overriden to support an enhanced comm
implementation.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Tested-by: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-wr683zwy94hmj4ibogmnv9ce@git.kernel.org


[ Rename thread__comm_curr() to thread__comm_str() ]
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
[ Fixed up some minor const pointer issues ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>