drm/nouveau: use nv_debug for NV_DEBUG, make DRM a separate subflag

It's really confusing for NV_DEBUG's printing to be controlled via
drm.debug while everything else is controlled via nouveau.debug. These
messages can be turned on with nouveau.debug=DRM=debug.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>