drivers/video: fsl-diu-fb: fix potential memcpy buffer overflow bug

It makes no sense to limit the size of a strncpy() to the length of
the source string.

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