drm/nouveau/gr: fallback to legacy paths during firmware lookup

Look for firmware files using the legacy ("nouveau/nvxx_fucxxxx") path
if they cannot be found in the new, "official" path. User setups were
broken by the switch, which is bad.

There are only 4 firmware files we may want to look up that way, so
hardcode them into the lookup function. All new firmware files should
use the standard "nvidia/<chip>/gr/" path.

Fixes: 8539b37a

 ("drm/nouveau/gr: use NVIDIA-provided external firmwares")
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org