drm: add SPDX idenitifier and clarify license

This is dual licensed under GPL-2.0 or MIT.

Signed-off-by: Dirk Hohndel (VMware) <dirk@hohndel.org>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: David Airlie <airlied@linux.ie>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180506231626.115996-2-dirk@hohndel.org

drivers/gpu/drm/drm_vma_manager.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: GPL-2.0 OR MIT
 /*
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA
  * Copyright (c) 2012 David Airlie <airlied@linux.ie>