kern/49495: serial console prevents use of X with drmkms

[prlw1%cam.ac.uk@localhost]

>Number:         49495
>Category:       kern
>Synopsis:       serial console prevents use of X with drmkms
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Dec 22 17:35:01 +0000 2014
>Originator:     Patrick Welche
>Release:        NetBSD-7.99.3/amd64
>Organization:
>Environment:
18 December -current code (but the issue is old - I don't remember ever being able to use a serial console and X with drmkms)
>Description:
Given a working -current + native X, try "consdev com0" at the boot
prompt. Now the monitor will remain blank. Everything seems to be in a "working" state - X process and xdm are running, no errors in messages
nor Xorg.0.log. Just no visible output.

(Happens to be an ivy bridge.)

>How-To-Repeat:

>Fix: