drm/nouveau/gr/gf100: split gf100_gr_init_ctxctl()

gf100_gr_init_ctxctl() is basically two different functions (one for
use of internal firmware, the other for use of external firmware), but
its current layout makes it look more complex than it is. Split it to
better reflect that fact.

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