drivers/video: fsl-diu-fb: remove unusued MEM_ALLOC_THRESHOLD

If there was ever any code that used MEM_ALLOC_THRESHOLD, it was removed a
long time ago.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>