Comment # 2 on bug 965536 from Dr. Werner Fink

OK just a try:

tux:~ # cat test.xinitrc 
#!/bin/bash -x
xrandr --listproviders
xrandr --setprovideroutputsource modesetting NVIDIA-0
xrandr --auto
xrandr -q
xterm
tux:~ # xinit ./test.xinitrc -- /usr/bin/X :1 vt8

leads to

+ xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x1f6; cap: 0x1 (Source Output); crtcs: 4; outputs: 2;
associated providers: 0; name: NVIDIA-0
    output DP-0
    output DP-1
Provider 1: id: 0x47; cap: 0x2 (Sink Output); crtcs: 3; outputs: 5; associated
providers: 0; name: modesetting
    output 0x42
    output 0x43
    output 0x44
    output 0x45
    output 0x46
+ xrandr --setprovideroutputsource modesetting NVIDIA-0
+ xrandr --auto
xrandr: Configure crtc 4 failed
X Error of failed request:  BadValue (integer parameter out of range for
operation)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  21 (RRSetCrtcConfig)
  Value in failed request:  0x0
  Serial number of failed request:  61
  Current serial number in output stream:  61
+ xrandr -q
Screen 0: minimum 8 x 8, current 1920 x 1080, maximum 16384 x 16384
DP-0 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis)
381mm x 214mm
   1920x1080      60.0*+
DP-1 disconnected (normal left inverted right x axis y axis)
eDP-1-0 connected
   1920x1080      60.0 +
   1920x1080_60   60.0  
   1400x1050      60.0  
   1280x1024      60.0  
   1280x960       60.0  
   1024x768       60.0     60.0  
   960x720        60.0  
   928x696        60.1  
   896x672        60.0  
   800x600        60.0     60.3     56.2  
   700x525        60.0  
   640x512        60.0  
   640x480        60.0     59.9  
   512x384        60.0  
   400x300        60.3     56.3  
   320x240        60.1  
DisplayPort-1-0 disconnected
HDMI-1-0 disconnected
DisplayPort-1-1 disconnected
HDMI-1-1 disconnected
  1920x1080 (0x48)  149.8MHz
        h: width  1920 start 1968 end 2068 total 2228 skew    0 clock   67.2KHz
        v: height 1080 start 1083 end 1084 total 1120           clock   60.0Hz
+ xterm