video: fbdev: pxa3xx-gcu: release the resources correctly in pxa3xx_gcu_probe/remove()

In pxa3xx_gcu_probe(), the sequence of error lable is wrong, it will
leads some resource leaked, so adjust the sequence to handle the error
correctly, and if pxa3xx_gcu_add_buffer() fails, pxa3xx_gcu_free_buffers()
need be called.
In pxa3xx_gcu_remove(), add missing clk_disable_unpreprare().

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Helge Deller <deller@gmx.de>