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: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/ttm/ttm_agp_backend.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_bo.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_bo_manager.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2007-2010 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_bo_util.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2007-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_bo_vm.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_execbuf_util.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_lock.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2007-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_memory.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_module.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_object.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2009-2013 VMware, Inc., Palo Alto, CA., USA

drivers/gpu/drm/ttm/ttm_tt.c

+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
 /**************************************************************************
  *
  * Copyright (c) 2006-2009 VMware, Inc., Palo Alto, CA., USA