drm: Prevent NULL deref in drm_name_info()

If a driver does not have a parent, or never sets the unique name for
itself, then we may proceed to chase a NULL dereference through
debugfs/.../name.

Testcase: igt/vgem_basic/debugfs
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466448813-23340-1-git-send-email-chris@chris-wilson.co.uk