drm/nouveau/drm/nouveau: Fix runtime PM leak in nv50_disp_atomic_commit()

A CRTC being enabled doesn't mean it's on! It doesn't even necessarily
mean it's being used. This fixes runtime PM leaks on the P50 I've got
next to me.

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