intel: Import intel_decode.c from intel-gpu-tools.

This is from commit dd9a5b4f.
We've been sharing this file between that repo and Mesa, and it's time
to build a real interface using it.  I'm also hoping to apply some of
its packet-walking logic for AUB dumping and batch validation
purposes.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Eugeni Dodonov <eugeni@dodonov.net>