how to use high resolution display with radeon

[Cheng Yutong <chyt-xyz@outlook.com>]

hello everyone,

I'm using an old graphic card on my debian PC, which is HD 6770,

and these days i've got a new display that support 2k resolution 
(2560x1440),

but on the Display Setting dialog, btw, my DE is XFCE4, the highest 
resolution is 1920x1200.

i've tried to generate a Modeline with `cvt 2560 1440`,

```

# 2560x1440 59.96 Hz (CVT 3.69M9) hsync: 89.52 kHz; pclk: 312.25 MHz
Modeline "2560x1440_60.00"  312.25  2560 2752 3024 3488  1440 1443 1448 
1493 -hsync +vsync
```

and add it to the display, but when set it with `xrandr`, it does not 
work well, i got an error message:

```

Input Signal Out ofRange!

Current Mode:

    H = 89.6kHz V = 60Hz

Change Mode to:

    1920 x 1200 60Hz

```

and i've installed the `firmware-amd-graphics`

`firmware-amd-graphics/stable,stable,now 20190114-2 all [installed]`.

could someone give some help?