drm backport from drm-next a3483353

Sync to commit:

commit a3483353
Author:     David Herrmann <dh.herrmann@gmail.com>
AuthorDate: Wed Nov 13 11:42:26 2013 +0100
Commit:     Dave Airlie <airlied@gmail.com>
CommitDate: Fri Nov 15 20:49:02 2013 +1000

    drm: check for !kdev in drm_unplug_minor()

Signed-off-by: Rob Clark <robdclark@gmail.com>