Misc: Add copyright/license statement to applicable files

Should've did this in the beginning.

src/common/d3d12/context.cpp

@@ -1,6 +1,6 @@
-// Copyright 2019 Dolphin Emulator Project
-// Licensed under GPLv2+
-// Refer to the license.txt file included.
+// SPDX-FileCopyrightText: 2019-2022 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: (GPL-3.0 OR CC-BY-NC-ND-4.0)
+// Parts originally from Dolphin Emulator, also written by myself.
 
 #include "context.h"
 #include "../assert.h"