drm/nouveau/drm/nouveau: Don't forget to cancel hpd_work on suspend/unload

Currently, there's nothing in nouveau that actually cancels this work
struct. So, cancel it on suspend/unload. Otherwise, if we're unlucky
enough hpd_work might try to keep running up until the system is
suspended.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>