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.

Fixes: ae1aadb1

 ("nouveau: don't fail driver load if no display hw present.")
Link: https://gitlab.freedesktop.org/drm/nouveau/-/issues/337
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Reviewed-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240607221032.25918-1-anarsoul@gmail.com