perf completion: Use more comp words

The completion words $words and $cword are available, so we might as
well use them instead of directly accessing COMP_WORDS.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1372941691-14684-8-git-send-email-artagnon@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>