When there is explicit configuration, it's better to use
confScreen->screennum instead of hardcoded 0.

When there is no configuration (default case) the screen number is
still 0 so it doesn't change behaviour.

But at least for a case when the Intel device is backed by the
intel driver and using a screen description like this below,
both providers are correctly assigned to :0.2.

  Section "Screen"
        Identifier      "SCREEN2"
        Option          "AutoServerLayout" "on"
        Device          "UDL"
        GPUDevice       "Intel2"
        Monitor         "Monitor-DVI-I-1"
        SubSection      "Display"
                Modes   "1024x768"
                Depth   24
        EndSubSection
  EndSection

  Section "ServerLayout"
        Identifier      "LAYOUT"
        Option          "AutoServerLayout" "on"
        Screen          0 "SCREEN"
        Screen          1 "SCREEN1" RightOf "SCREEN"
        Screen          2 "SCREEN2" RightOf "SCREEN1"
  EndSection

  # DISPLAY=:0.2 xrandr --listproviders
  Providers: number : 2
  Provider 0: id: 0xd2 cap: 0x2, Sink Output crtcs: 1 outputs: 1 associated providers: 0 name:modesetting
  Provider 1: id: 0xfd cap: 0xb, Source Output, Sink Output, Sink Offload crtcs: 2 outputs: 2 associated providers: 0 name:Intel

Signed-off-by: Zoltán Böszörményi <zboszor@gmail.com>