drm/nouveau: fix blank LVDS screen regression on pre-nv50 cards

Commit 2a44e499 ("drm/nouveau/disp: introduce proper init/fini, separate
from create/destroy") started to call display init routines on pre-nv50
hardware on module load. But LVDS init code sets driver state in a way
which prevents modesetting code from operating properly.

nv04_display_init calls nv04_dfp_restore, which sets encoder->last_dpms to
NV_DPMS_CLEARED.

drm_crtc_helper_set_mode
  nv04_dfp_prepare
    nv04_lvds_dpms(DRM_MODE_DPMS_OFF)

nv04_lvds_dpms checks last_dpms mode (which is NV_DPMS_CLEARED) and wrongly
assumes it's a "powersaving mode", the new one (DRM_MODE_DPMS_OFF) is too,
so it skips calling some crucial lvds scripts.
Reported-by: Chris Paulson-Ellis <chris@edesix.com>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/nouveau/nv04_dfp.c

@@ -505,7 +505,7 @@ static void nv04_dfp_update_backlight(struct drm_encoder *encoder, int mode)
 
 static inline bool is_powersaving_dpms(int mode)
 {
-	return (mode != DRM_MODE_DPMS_ON);
+	return mode != DRM_MODE_DPMS_ON && mode != NV_DPMS_CLEARED;
 }
 
 static void nv04_lvds_dpms(struct drm_encoder *encoder, int mode)