drm/nv50: fix smatch warning in nv50_vram.c

Reported-by: NDan Carpenter <error27@gmail.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nv50_vm.c

@@ -91,7 +91,8 @@ void
 nv50_vm_map(struct nouveau_vma *vma, struct nouveau_gpuobj *pgt,
 	    struct nouveau_vram *mem, u32 pte, u32 cnt, u64 phys)
 {
-	u32 block, i;
+	u32 block;
+	int i;
 
 	phys  = nv50_vm_addr(vma, pgt, phys, mem->memtype, 0);
 	pte <<= 3;