drm: add SPDX identifier and clarify license

This is dual licensed under GPL-2.0 or MIT.
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dirk Hohndel (VMware) <dirk@hohndel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180506231626.115996-4-dirk@hohndel.org

drivers/gpu/drm/drm_global.c

+// SPDX-License-Identifier: GPL-2.0 OR MIT
 /**************************************************************************
  *
  * Copyright 2008-2009 VMware, Inc., Palo Alto, CA., USA
- * All Rights Reserved.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the