drm/nouveau/gr/gk20a: move firmware bundle release to gf100

Some members of gf100_gr were freed by the gk20a driver. That's not
where it should be done - free them in gf100 so other chips that use
NVIDIA-provided firmware free these structures properly.

This also removes the need for a GK20A-specific destructor.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>