drm/nvc0/gr: fix gpc firmware regression

"drm/nve0-/gr: some new gpc registers can have multiple copies"
5ee86c4190f9e caused a regression for nvc0, because the bit indicating last
transfer has occured was no longer set, resulting in random system lockups.

Reported-by: Ronald Uitermark <ronald645@gmail.com>
Tested-by: Ronald Uitermark <ronald645@gmail.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>