drm/nouveau/gr/gf100-: fix race condition in fecs/gpccs ucode

This is a simplied version of the fix by Roy in fdo#93629.  While this
doesn't appear to fix the issues for the users in that report, it's a
real issue that deserves to be resolved.

Reported-by: Roy Spliet <rspliet@eclipso.eu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>