drm/nouveau: gr/gk20a: Use firmware version 0

Tegra firmware doesn't actually use any version numbers and passing -1
causes the existing firmware binaries not to be found. Use version 0 to
find the correct files.

Fixes: ef16dc27 ("drm/nouveau/gr/gf100-: select implementation based on available FW")
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>