neofb: reduce panning function

Reduce panning function by deleting checks done by higher layer and
folding remaining function into the called one.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>