nixos/xserver: inform xserver about externally configured drivers

This commit adds a new (internal) option to let the xserver module know
about externally configured drivers (i.e., drivers that this module
should treat as "configured" but otherwise ignore). Without this, the
xserver module will think the driver hasn't been configured and throw an
error.

This commit also expands the documentation to explain when to choose
`drivers` versus `externallyConfiguredDrivers`.

nixos/modules/services/x11/xserver.nix

@@ -497,7 +497,25 @@ in
         internal = true;
         description = ''
           A list of attribute sets specifying drivers to be loaded by
-          the X11 server.
+          the X11 server. This module will create a Device section in
+          the Xorg config for every driver listed here, along with a
+          Screen section if the driver's {option}`display` attribute is
+          `true`. If such configuration sections should not be created,
+          use {options}`services.xserver.externallyConfiguredDrivers`
+          instead.
+
+          Users should not add drivers to this option but should instead
+          add drivers to {option}`services.xserver.videoDrivers`.
+        '';
+      };
+
+      externallyConfiguredDrivers = mkOption {
+        type = types.listOf types.str;
+        internal = true;
+        description = ''
+          A list of externally configured drivers (by name). Modules that
+          manually configure their drivers should add said drivers to this
+          list to let this module know that the driver has been configured.
         '';
       };
 
@@ -865,7 +883,9 @@ in
       }
     ]
     ++ map (driver: {
-      assertion = builtins.elem driver (builtins.catAttrs "name" cfg.drivers);
+      assertion = builtins.elem driver (
+        (builtins.catAttrs "name" cfg.drivers) ++ cfg.externallyConfiguredDrivers
+      );
       message = "Unknown X11 driver ‘${driver}’ specified in `services.xserver.videoDrivers`.";
     }) cfg.videoDrivers;