bin/53891: sandy bridge i965 SNA bus error

[prlw1%cam.ac.uk@localhost]

>Number:         53891
>Category:       bin
>Synopsis:       sandy bridge i965 SNA bus error
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Jan 18 16:55:00 +0000 2019
>Originator:     Patrick Welche
>Release:        NetBSD-8.99.30/amd64
>Organization:
>Environment:
>Description:
NetBSD/amd64 laptop was stable using SNA for years. After the recent xorg-server update, X dumps unusable cores, with e.g. SEGV, ABRT or BUS. See http://mail-index.netbsd.org/current-users/2019/01/04/msg034891.html thread for examples.

Finally a clue: reverting to UXA gives a stable system.

[Xorg.0.log and dmesg to follow once I have a PR number.]
>How-To-Repeat:

>Fix: