drm/nouveau/fb/gk20a: use regular gf100's functions

gk20a's FB is not special compared to other Kepler chips, besides the
fact it does not have VRAM. Use the regular gf100 hooks instead of the
incomplete versions we rewrote.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-By: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>