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.