drm/nouveau: don't attempt to schedule hpd_work on headless cards

In the Linux kernel, the following vulnerability has been resolved: drm/nouveau: don't attempt to schedule hpd_work on headless cards If the card doesn't have display hardware, hpd_work and hpd_lock are left uninitialized which causes BUG when attempting to schedule hpd_work on runtime PM resume. Fix it by adding headless flag to DRM and skip any hpd if it's set.