drivers/video/mx3fb.c: use NULL for pointer

Fix the following sparse error:

  drivers/video/mx3fb.c:1309:28: warning: Using plain integer as NULL pointer

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>