drm/nouveau/fb/gp102-: unlock VPR right after devinit

Under memory load, instmem allocations could end up in the regions of
VRAM that are inaccessible right after boot, and be corrupted after a
suspend/resume cycle as a result of being restored before booting the
mem unlock firmware.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>