drm/nouveau/pm: allow voltage-only perflvl set, enable nvc0

Okay, my card didn't blow up.  Lets turn it on!

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nouveau_pm.c b/drivers/gpu/drm/nouveau/nouveau_pm.c
index 53d2ad8..179067a 100644
--- a/drivers/gpu/drm/nouveau/nouveau_pm.c
+++ b/drivers/gpu/drm/nouveau/nouveau_pm.c
@@ -72,10 +72,12 @@
 		}
 	}
 
-	nouveau_pm_clock_set(dev, perflvl, PLL_CORE, perflvl->core);
-	nouveau_pm_clock_set(dev, perflvl, PLL_SHADER, perflvl->shader);
-	nouveau_pm_clock_set(dev, perflvl, PLL_MEMORY, perflvl->memory);
-	nouveau_pm_clock_set(dev, perflvl, PLL_UNK05, perflvl->unk05);
+	if (pm->clock_set) {
+		nouveau_pm_clock_set(dev, perflvl, PLL_CORE, perflvl->core);
+		nouveau_pm_clock_set(dev, perflvl, PLL_SHADER, perflvl->shader);
+		nouveau_pm_clock_set(dev, perflvl, PLL_MEMORY, perflvl->memory);
+		nouveau_pm_clock_set(dev, perflvl, PLL_UNK05, perflvl->unk05);
+	}
 
 	pm->cur = perflvl;
 	return 0;
@@ -92,9 +94,6 @@
 	if (nouveau_perflvl_wr != 7777)
 		return -EPERM;
 
-	if (!pm->clock_set)
-		return -EINVAL;
-
 	if (!strncmp(profile, "boot", 4))
 		perflvl = &pm->boot;
 	else {