drm/nouveau/fb/gk20a: fix constructor call

The gf100 constructor should be called, otherwise we will allocate a
smaller object than expected. This was without effect so far because
gk20a did not allocate a page, but with gf100's page allocation moved
to the oneinit() hook this problem has become apparent.

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