drm/nouveau/pmu: do not assume a PMU is present

Some devices may not have a PMU. Avoid a NULL pointer dereference in
such cases by checking whether the pointer given to nvkm_pmu_pgob() is
valid.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/pmu/base.c

@@ -28,7 +28,7 @@
 void
 nvkm_pmu_pgob(struct nvkm_pmu *pmu, bool enable)
 {
-	if (pmu->func->pgob)
+	if (pmu && pmu->func->pgob)
 		pmu->func->pgob(pmu, enable);
 }