powerpc/6xx: add missing iounmap() on error in hlwd_pic_init()

Add the missing iounmap() before return from hlwd_pic_init()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Scott Wood <scottwood@freescale.com>