drm/nouveau/fb/gp102-: unlock VPR as part of FB init

We perform memory allocations long before we hit the code in SECBOOT that
would unlock the VPR, which could potentially result in memory allocation
within the locked region.

Run the scrubber binary right after VRAM init to ensure we don't.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>