drm/sysfs: fix OOM verification

Copy/Paste typo.. we need to test for ->kdev instead of ->dev.

Reported-by: Juha Leppänen <juha_efku@dnainternet.net>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>