drm/nouveau/fb/gf100-: Fix 32 bit wraparound in new ram detection

A missing u64 cast causes a 32-Bit wraparound from
4096 MiB to 0 MiB and therefore total 0 MiB VRAM detected
if card has 4096 Mib per FBP.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>