fbdev: nv: fix sparse noise

Mostly signedness fixes.  nv10_sim_state existence in both drivers suggests
that one of them should be removed, but that's for later.

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