drm/udl: make usage as a console safer

commit bcb39af4486be07e896fc374a2336bad3104ae0a upstream.

Okay you don't really want to use udl devices as your console, but if
you are unlucky enough to do so, you run into a lot of schedule while atomic
due to printk being called from all sorts of funky places. So check if we
are in an atomic context, and queue the damage for later, the next printk
should cause it to appear. This isn't ideal, but it is simple, and seems to
work okay in my testing here.

(dirty area idea came from xenfb)

fixes a bunch of sleeping while atomic issues running fbcon on udl devices.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>