drm/nouveau/nvif: fix null ptr deref on pre-fermi boards

drm/nouveau/nvif: fix null ptr deref on pre-fermi boards

Check that gpfifo.post() exists before trying to call it.

Fixes: 862450a85b85 ("drm/nouveau/gf100-: track chan progress with non-WFI semaphore release")
Reported-by: Jamie Heilman <jamie@audible.transient.net>
Closes: https://lore.kernel.org/lkml/aElJIo9_Se6tAR1a@audible.transient.net/
Reported-by: Rui Salvaterra <rsalvaterra@gmail.com>
Closes: https://lore.kernel.org/all/CALjTZvZgH0N43rMTcZiDVSX93PFL680hsYPwtp8=Ja1OWPvZ1A@mail.gmail.com/
Tested-by: Rui Salvaterra <rsalvaterra@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Link: https://lore.kernel.org/r/20250714025923.29591-1-bskeggs@nvidia.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>