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