drivers/video/jz4740_fb.c: use devm_ functions

The various devm_ functions allocate memory that is released when a driver
detaches. This patch uses these functions for data that is allocated in the
probe function of a platform device and is only freed in the remove function.

Signed-off-by: Damien Cassou <damien.cassou@lifl.fr>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>