tools/intel_gpu_top: Add file output capability

A new -o command switch enables logging to a file.

v2:
 * Support "-o -" for explicit stdout selection. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=108689


Cc: Eero Tamminen <eero.t.tamminen@intel.com>
Cc: 3.14pi@ukr.net
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>