drm: check for !kdev in drm_unplug_minor()

We moved minor deallocation to drm_dev_free() in:
  commit 8f6599da
  Author: David Herrmann <dh.herrmann@gmail.com>
  Date:   Sun Oct 20 18:55:45 2013 +0200

      drm: delay minor destruction to drm_dev_free()

However, this causes a call to drm_unplug_minor(), which should just do
nothing as drm_dev_unregister() already called this.

But a separate patch caused kdev lifetime changes:
  commit 5bdebb18


  Author: Dave Airlie <airlied@redhat.com>
  Date:   Fri Oct 11 14:07:25 2013 +1000

      drm/sysfs: sort out minor and connector device object lifetimes.

Thus making our dev_is_registered() call useles (and even segfault if it
is NULL). Replace it with a simple !kdev test and we're fine.

Reported-by: Huax Lu <huax.lu@intel.com>
Reported-by: Daniel Vetter <daniel@ffwll.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71208


Tested-by: lu hua <huax.lu@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>