[Nouveau] Quadro 4000 with iiyama B2783QSU should be 2560x1440 but detected as 1920x1080.

[Roger Price]

Hi, Distribution: openSUSE 42.2 Beta, Kernel: 4.4.  The box is a Dell 
Precision T7500 with an nVidia Quadro 4000 card and a displayport 
connected iiyama B2783QSU 2560x1440 monitor identified as DP-1.

Problem: The monitor is detected as 1920x1080 and not 2560x1440.  See Xorg 
log file at http://rogerprice.org/Xorg.0.log .  There is no file 
/etc/X11/xorg.conf and in the directory /etc/X11/Xorg.conf.d/ the files 
50-device.conf, 50-monitor.conf and 50-screen.conf contain comments only.

My attempted repair failed:

I used gtf to calculate a 2560x1440_60 mode line

   Modeline "2560x1440_60.00"  311.83  2560 2744 3024 3488  1440 1441 1444 1490  -HSync +Vsync

and I then used commands

   xrandr --newmode 2560x1440   311.83  2560 2744 3024 3488  1440 1441 1444 1490  -HSync +Vsync
   xrandr --addmode DP-1 2560x1440
   xrandr --output DP-1 --mode 2560x1440

The result is that windows on the screen seem to increase in size slightly 
and flow off the screen.  The system tray normally at the bottom of the 
screen has disappeared.  More precisely, command xrandr -q reports:

   Screen 0: minimum 320 x 200, current 2560 x 1440, maximum 16384 x 16384
   DVI-I-1 disconnected (normal left inverted right x axis y axis)
   DP-1 connected 2560x1440+0+0 (normal left inverted right x axis y axis) 597mm x 336mm
     1920x1200     59.95
     1920x1080     60.00
     1600x1200     60.00
     1680x1050     59.88
     1600x1000     60.01
     1600x900      60.00
     1280x1024     75.02
     1440x900      59.90
     1024x768      75.03    60.00
     800x600       75.00    60.32
     640x480       75.00    59.94
     2560x1440     60.00*
   DP-2 disconnected (normal left inverted right x axis y axis)

but command xvidtune -show reports:

   "1920x1200"   154.00   1920 1968 2000 2080   1200 1203 1209 1235 +hsync -vsync

I'm not sure if I'm looking at a bug or if this is just noob confusion. 
Any advice or suggestion would be much appreciated.

Roger