drm/nouveau: fix backlight mask on ppc powerbook

This code was originally moved to using nv_mask by d31e078d. This
should not have any actual effect since the mask isn't applied to the
value.
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/dispnv04/dfp.c

@@ -493,7 +493,7 @@ static void nv04_dfp_update_backlight(struct drm_encoder *encoder, int mode)
 	if (dev->pdev->device == 0x0174 || dev->pdev->device == 0x0179 ||
 	    dev->pdev->device == 0x0189 || dev->pdev->device == 0x0329) {
 		if (mode == DRM_MODE_DPMS_ON) {
-			nv_mask(device, NV_PBUS_DEBUG_DUALHEAD_CTL, 0, 1 << 31);
+			nv_mask(device, NV_PBUS_DEBUG_DUALHEAD_CTL, 1 << 31, 1 << 31);
 			nv_mask(device, NV_PCRTC_GPIO_EXT, 3, 1);
 		} else {
 			nv_mask(device, NV_PBUS_DEBUG_DUALHEAD_CTL, 1 << 31, 0);