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
(primarily header files) were simply missing the boiler plate and got
caught up in the global update.

Fixes: b2441318 (License cleanup: add SPDX GPL-2.0 license identifier to files with no license)
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>