media: staging/intel-ipu3: Fix race condition during set_fmt
Do not modify imgu_pipe->nodes[inode].vdev_fmt.fmt.pix_mp, until the
format has been correctly validated.
Otherwise, even if we use a backup variable, there is a period of time
where imgu_pipe->nodes[inode].vdev_fmt.fmt.pix_mp might have an invalid
value that can be used by other functions.