drm/nouveau: fix suspend/resume when in headless mode

commit 9430738d80223a1cd791a2baa74fa170d3df1262 upstream.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
[jrnieder@gmail.com: backport to 3.2: make fbcon suspend/resume
 handling conditional in a vague hope that this will approximate what
 the original does for 3.3+]
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>