drm/amdgpu: Potential uninitialized variable in amdgpu_vm_update_directories()

After commit ea09729c ("drm/amdgpu: rework page directory filling
v2") then it becomes a lot harder to verify that "r" is initialized.  My
static checker complains and so I've reviewed the code.  It does look
like it might be buggy... Anyway, it doesn't hurt to set "r" to zero
at the start.
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -1244,7 +1244,7 @@ static void amdgpu_vm_invalidate_level(struct amdgpu_vm *vm,
 int amdgpu_vm_update_directories(struct amdgpu_device *adev,
 				 struct amdgpu_vm *vm)
 {
-	int r;
+	int r = 0;
 
 	spin_lock(&vm->status_lock);
 	while (!list_empty(&vm->relocated)) {