drm/nouveau/mxm: initial implementation of dcb sanitisation

The DCB table provided by the VBIOS on most MXM chips has a number of
entries which either need to be disabled, or modified according to the
MXM-SIS Output Device Descriptors.

The x86 vbios code usually takes care of this for us, however, with the
large number of laptops now with switchable graphics or optimus, a lot
of the time nouveau is responsible for POSTing the card instead - leaving
some fun situations like, plugging in a monitor and having nouveau decide
3 connectors actually just got plugged in..

No MXM-SIS fetching methods implemented yet.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>