drivers/video/gbefb.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>