drm: cleanup debugfs in drm_unplug_minor()

There is no reason to delay debugfs-cleanup to drm_put_minor(). We should
forbid any access to debugfs files once the device is dead. Chances they
oops once a card was unplugged are very high, anyway.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>