xf86: autobind GPUs to the screen

This is a modified version of a patch we've been carry-ing in Fedora and RHEL for years now. This patch automatically adds secondary GPUs to the master as output sink / offload source making e.g. the use of slave-outputs just work, with requiring the user to manually run "xrandr --setprovideroutputsource" before he can hookup an external monitor to his hybrid graphics laptop.

There is one problem with this patch, which is why it was not upstreamed before. What to do when a secondary GPU gets detected really is a policy decission (e.g. one may want to autobind PCI GPUs but not USB ones) and as such should be under control of the Desktop Environment.

Unconditionally adding autobinding support to the xserver will result in races between the DE dealing with the hotplug of a secondary GPU and the server itself dealing with it.

However we've waited for years for any Desktop Environments to actually start doing some sort of autoconfiguration of secondary GPUs and there is still not a single DE dealing with this, so I believe that it is time to upstream this now.

To avoid potential future problems if any DEs get support for doing secondary GPU configuration themselves, the new autobind functionality is made optional. Since no DEs currently support doing this themselves it is enabled by default. When DEs grow support for doing this themselves they can disable the servers autobinding through the servers cmdline or a xorg.conf snippet.