Allow to output statistics to stdout.

Allow to output statistics to stdout.

This allows intel_gpu_top to run both in statistics-collecting mode
(collecting the per-ring statistics in gnuplot-friendly format) and
ncurses top-like mode at the same time.

It also allows to output the statistics directly to stdout, by using "-o
-", so the results can be parsed directly via a popen() parsing.

If you are using intel_gpu_top as previously (without any command-line
arguments), it should change nothing for you. If you were using its
logging facilities (e.g., the '-o file'), note that the logging will keep
running, but the detailed top-like interface will be on the screen at the
same time.

Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>