ARM: pxa168fb: add .remove function

The pxa168fb driver is missing .remove function so the framebuffer isn't
correctly shut down when the module is removed.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>