freescale DIU: bug fix: add sanity check for AOI position

AOI position cannot be negative.

Signed-off-by: York Sun <yorksun@freescale.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Cc: Krzysztof Helt <krzysztof.h1@poczta.fm>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>