drm: nouveau: fix disp.c build when NOUVEAU_BACKLIGHT is not enabled

Fix build errors and warnings when CONFIG_DRM_NOUVEAU_BACKLIGHT is not set

../drivers/gpu/drm/nouveau/dispnv50/disp.c: In function ‘nv50_sor_atomic_disable’:
../drivers/gpu/drm/nouveau/dispnv50/disp.c:1665:52: error: ‘struct nouveau_connector’ has no member named ‘backlight’
  struct nouveau_backlight *backlight = nv_connector->backlight;
                                                    ^~
../drivers/gpu/drm/nouveau/dispnv50/disp.c:1670:28: error: dereferencing pointer to incomplete type ‘struct nouveau_backlight’
  if (backlight && backlight->uses_dpcd) {

and then fix subsequent build warnings after the above are fixed:

../drivers/gpu/drm/nouveau/dispnv50/disp.c: In function ‘nv50_sor_atomic_disable’:
../drivers/gpu/drm/nouveau/dispnv50/disp.c:1669:6: warning: unused variable ‘ret’ [-Wunused-variable]
  int ret;
      ^~~
../drivers/gpu/drm/nouveau/dispnv50/disp.c:1662:22: warning: unused variable ‘drm’ [-Wunused-variable]
  struct nouveau_drm *drm = nouveau_drm(nv_encoder->base.base.dev);
                      ^~~

Fixes: 6eca310e ("drm/nouveau/kms/nv50-: Add basic DPCD backlight support for nouveau")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714171523.413-1-rdunlap@infradead.org

drivers/gpu/drm/nouveau/dispnv50/disp.c

@@ -1659,23 +1659,27 @@ static void
 nv50_sor_atomic_disable(struct drm_encoder *encoder, struct drm_atomic_state *state)
 {
 	struct nouveau_encoder *nv_encoder = nouveau_encoder(encoder);
-	struct nouveau_drm *drm = nouveau_drm(nv_encoder->base.base.dev);
 	struct nouveau_crtc *nv_crtc = nouveau_crtc(nv_encoder->crtc);
 	struct nouveau_connector *nv_connector = nv50_outp_get_old_connector(state, nv_encoder);
+#ifdef CONFIG_DRM_NOUVEAU_BACKLIGHT
+	struct nouveau_drm *drm = nouveau_drm(nv_encoder->base.base.dev);
 	struct nouveau_backlight *backlight = nv_connector->backlight;
+#endif
 	struct drm_dp_aux *aux = &nv_connector->aux;
 	int ret;
 	u8 pwr;
 
+#ifdef CONFIG_DRM_NOUVEAU_BACKLIGHT
 	if (backlight && backlight->uses_dpcd) {
 		ret = drm_edp_backlight_disable(aux, &backlight->edp_info);
 		if (ret < 0)
 			NV_ERROR(drm, "Failed to disable backlight on [CONNECTOR:%d:%s]: %d\n",
 				 nv_connector->base.base.id, nv_connector->base.name, ret);
 	}
+#endif
 
 	if (nv_encoder->dcb->type == DCB_OUTPUT_DP) {
-		int ret = drm_dp_dpcd_readb(aux, DP_SET_POWER, &pwr);
+		ret = drm_dp_dpcd_readb(aux, DP_SET_POWER, &pwr);
 
 		if (ret == 0) {
 			pwr &= ~DP_SET_POWER_MASK;