drm: Show leaked connectors upon unload

After warning that the connector list is not empty on device
unregistration (i.e. module unload) also print out which connectors are
still hanging around to aide finding the leak.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170119090513.4154-1-chris@chris-wilson.co.uk