drm/nouveau: convert bo.mem.start usage to bo.offset

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
diff --git a/drivers/gpu/drm/nouveau/nouveau_object.c b/drivers/gpu/drm/nouveau/nouveau_object.c
index ab4be9c..37e6ca8 100644
--- a/drivers/gpu/drm/nouveau/nouveau_object.c
+++ b/drivers/gpu/drm/nouveau/nouveau_object.c
@@ -768,7 +768,7 @@
 			struct nouveau_gpuobj *sem = NULL;
 			struct nv50_display_crtc *dispc =
 				&nv50_display(dev)->crtc[i];
-			u64 offset = dispc->sem.bo->bo.mem.start << PAGE_SHIFT;
+			u64 offset = dispc->sem.bo->bo.offset;
 
 			ret = nouveau_gpuobj_dma_new(chan, 0x3d, offset, 0xfff,
 						     NV_MEM_ACCESS_RW,