[media] media: davinci_vpfe: fix missing unlock on error in vpfe_prepare_pipeline()

Add the missing unlock before return from function
vpfe_prepare_pipeline() in the error handling case.

video->lock is lock/unlock in function vpfe_open(),
and no need to unlock it here, so remove unlock
video->lock.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>