function-graph: consolidate prologues for output

Impact: clean up

The prologue of the function graph entry, return and comments all
start out pretty much the same. Each of these duplicate code and
do so slightly differently.

This patch consolidates the printing of the pid, absolute time,
cpu and proc (and for entry, the interrupt).

Signed-off-by: Steven Rostedt <srostedt@redhat.com>