Update contrast_preserve.hpp

Update contrast_preserve.hpp

fix a build warning:

```
C:\Slave\workspace\precommit\windows10\opencv\modules\photo\src\contrast_preserve.hpp(289): warning C4244: '=': conversion from 'double' to '_Tp', possible loss of data
        with
        [
            _Tp=float
        ]
C:\Slave\workspace\precommit\windows10\opencv\modules\photo\src\contrast_preserve.hpp(361): warning C4244: '=': conversion from 'double' to '_Tp', possible loss of data
        with
        [
            _Tp=float
        ]
```

(from https://build.opencv.org.cn/job/precommit/job/windows10/1633/console)