fbdev: gbefb: Convert to use dev_groups

The driver core supports the ability to handle the creation and removal
of device-specific sysfs files in a race-free manner. Moreover, it can
guarantee the success of creation. Therefore, it should be better to
convert to use dev_groups.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: Helge Deller <deller@gmx.de>