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I have a Debian Stretch laptop with two graphics controllers:

[0]~>lspci | fgrep 'VGA compatible controller' 

00:02.0 VGA compatible controller: Intel Corporation Iris Pro Graphics P580 (rev 09)

01:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Venus XTX [Radeon HD 8890M / R9 M275X/M375X] (rev 83)

Now the Intel controller seems to be used for GLX rendering:

[0]~>glxinfo | fgrep Device:

Device: Mesa DRI Intel(R) Iris Pro Graphics P580 (Skylake GT4e)  (0x193d)

[0]~>glxgears -info | fgrep GL_RENDERER

GL_RENDERER   = Mesa DRI Intel(R) Iris Pro Graphics P580 (Skylake GT4e)

[0]~>glmark2 | fgrep GL_RENDERER:

GL_RENDERER:   Mesa DRI Intel(R) Iris Pro Graphics P580 (Skylake GT4e) 

These modules are loaded:

[0]~>fgrep LoadModule: /var/log/Xorg.0.log 

[    10.301] (II) LoadModule: "glx"

[    10.312] (II) LoadModule: "ati"

[    10.312] (II) LoadModule: "radeon"

[    10.314] (II) LoadModule: "modesetting"

[    10.315] (II) LoadModule: "fbdev"

[    10.316] (II) LoadModule: "vesa"

[    10.349] (II) LoadModule: "fbdevhw"

[    10.350] (II) LoadModule: "glamoregl"

[    10.987] (II) LoadModule: "fb"

[    10.988] (II) LoadModule: "fb"

[    10.988] (II) LoadModule: "dri2"

[    10.988] (II) LoadModule: "glamoregl"

[    12.980] (II) LoadModule: "ramdac"

[    13.195] (II) LoadModule: "libinput"

I have no X.Org configuration file /etc/X11/xorg.conf.

How can I configure X.Org to use the AMD controller?

Cool, both your GPU drivers can handle both output source/sink and render offload source/sink operations. So... try this:

xrandr --setprovideroffloadsink "VERDE @ pci:0000:01:00.0" "modesetting"

or by using provider numbers:

xrandr --setprovideroffloadsink 1 0

This tells the Intel controller to offload its rendering tasks to the AMD GPU. It is not as efficient as switching the outputs to the AMD GPU, but it's the easiest way to make use of both your GPUs with your current configuration.

If it works for you, put that command into a script in /etc/X11/Xsession.d directory, for example /etc/X11/Xsession.d/15custom_xrandr:

#!/bin/sh

xrandr -setprovideroffloadsink "VERDE @ pci:0000:01:00.0" "modesetting"

If you have the debugfs virtual filesystem mounted at /sys/kernel/debug and /sys/kernel/debug/vgaswitcheroo/ directory exists, you may be able to switch the outputs to the AMD GPU's control, if your hardware includes the required multiplexer chip. It should be as easy as arranging something to run

echo "DIS" >>/sys/kernel/debug/vgaswitcheroo/switch

before starting the X server. Then the X server should automatically detect that all the active displays are now on the AMD GPU, and should leave the Intel GPU alone.