drm/radeon: fix incorrrect SPDX-License-Identifiers

radeon is MIT.  This were incorrectly changed in
commit b2441318 ("License cleanup: add SPDX GPL-2.0 license identifier to files with no license")
and
commit d198b34f (".gitignore: add SPDX License Identifier")
and:
commit ec8f24b7 ("treewide: Add SPDX license identifier - Makefile/Kconfig")

Fixes: d198b34f (".gitignore: add SPDX License Identifier")
Fixes: ec8f24b7 ("treewide: Add SPDX license identifier - Makefile/Kconfig")
Fixes: b2441318 ("License cleanup: add SPDX GPL-2.0 license identifier to files with no license")
Bug: drm/amd#2053


Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>