drm/nouveau/ltc/gm107: use nvkm_mask to set cbc_ctrl1

resman and nvgpu both do this, presumably for good reason.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/ltc/gm107.c

@@ -32,7 +32,7 @@ gm107_ltc_cbc_clear(struct nvkm_ltc *ltc, u32 start, u32 limit)
 	struct nvkm_device *device = ltc->subdev.device;
 	nvkm_wr32(device, 0x17e270, start);
 	nvkm_wr32(device, 0x17e274, limit);
-	nvkm_wr32(device, 0x17e26c, 0x00000004);
+	nvkm_mask(device, 0x17e26c, 0x00000000, 0x00000004);
 }
 
 static void