drm/nouveau/gr/gk104-: prevent reading non-existent regs in intr handler

Under certain circumstances the trapped address will contain subc 7,
which GK104 GR doesn't have anymore.

Notice this case to avoid causing additional priv ring faults.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/engine/gr/gf100.c

@@ -1097,9 +1097,14 @@ gf100_gr_intr(struct nvkm_subdev *subdev)
 	u32 subc = (addr & 0x00070000) >> 16;
 	u32 data = nv_rd32(priv, 0x400708);
 	u32 code = nv_rd32(priv, 0x400110);
-	u32 class = nv_rd32(priv, 0x404200 + (subc * 4));
+	u32 class;
 	int chid;
 
+	if (nv_device(priv)->card_type < NV_E0 || subc < 4)
+		class = nv_rd32(priv, 0x404200 + (subc * 4));
+	else
+		class = 0x0000;
+
 	engctx = nvkm_engctx_get(engine, inst);
 	chid   = pfifo->chid(pfifo, engctx);