drm/nouveau: fix bogus GPL-2 license header

The bulk SPDX addition made all these files into GPL-2.0 licensed files.
However the remainder of the project is MIT-licensed, these files
were simply missing the boiler plate and got caught up in the global update.

Fixes: 96ac6d43

 (treewide: Add SPDX license identifier - Kbuild)
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>