demmt: multi-device/context support and automatic chipset detection

Yay, no more static data describing gpu state.

Notes:
- it works only on traces obtained by mmt built after 7 Sep,
- it works only for traces which use IBs (which is tesla+, but without some
  modesetting traces which use old-style (pre-nv50) command submission)
- it leaks memory (object state) on ioctl_destroy,
- nouveau support is a bunch of hacks