drm/nouveau/gr/gk20a,gm200-: add terminators to method lists read from fw

Method init is typically ordered by class in the FW image as ThreeD,
TwoD, Compute.

Due to a bug in parsing the FW into our internal format, we've been
accidentally sending Twod + Compute methods to the ThreeD class, as
well as Compute methods to the TwoD class - oops.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>