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