drm/nouveau/gr/gf100: load firmware in outer function

The firmwares required by GR may vary from chip to chip, especially with
the introduction of secure boot and NVIDIA-provided firmwares. Move the
firmware loading outside of gf100_gr_ctor so other chips may still call
it while managing their firmwares themselves.

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