drm/nouveau/kms: switch hpd_lock from mutex to spinlock

There's no good reason for this to be a mutex, and once the layers of
workqueues have been untangled, nouveau_connector_hpd() can be called
from IRQ context and won't be able to take a mutex.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>