nvidiafb: fix sparse warning

Fix the following sparse warning:

drivers/video/nvidia/nv_setup.c:659:20: warning: dereference of noderef expression

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>