drm/nv50: fix detection of second vram rank

Goes a long way to correcting NVS295 memory reclocking issues.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Martin Peres <martin.peres@labri.fr>

drivers/gpu/drm/nouveau/nv50_vram.c

@@ -207,7 +207,7 @@ nv50_vram_init(struct drm_device *dev)
 		break;
 	}
 
-	dev_priv->vram_rank_B = (nv_rd32(dev, NV04_PFB_CFG0) & 0x100) >> 8;
+	dev_priv->vram_rank_B = !!(nv_rd32(dev, 0x100200) & 0x4);
 	dev_priv->vram_size  = nv_rd32(dev, 0x10020c);
 	dev_priv->vram_size |= (dev_priv->vram_size & 0xff) << 32;
 	dev_priv->vram_size &= 0xffffffff00ULL;