drm/radeon: change SPDX identifier to MIT

Commit b2441318 added
"SPDX-License-Identifier: GPL-2.0" to files which previously had no
license, change this to MIT for radeon matching the license text of the
other radeon files.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>