Regression: xf86ScrnToScreen: Assertion `pScrn->scrnIndex < screenInfo.numScreens' failed

With the xserver master at 5315f988 on Ubuntu 19.04, Xorg fails to start with:

X.Org X Server 1.20.99.1
X Protocol Version 11, Revision 0
Current Operating System: Linux ccu-x230 5.0.0-31-generic #33-Ubuntu SMP Mon Sep 30 18:51:59 UTC 2019 x86_64
Kernel command line: BOOT_IMAGE=/vmlinuz-5.0.0-31-generic root=/dev/mapper/ccu--x230--vg-root ro splash quiet vt.handoff=1
 
Current version of pixman: 0.36.0
        Before reporting problems, check http://wiki.x.org
        to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
        (++) from command line, (!!) notice, (II) informational,
        (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/home/pq/jhbuild/install/var/log/Xorg.0.log", Time: Mon Oct 14 12:25:03 2019
(==) Using system config directory "/home/pq/jhbuild/install/share/X11/xorg.conf.d"
Xorg: ../hw/xfree86/common/xf86Helper.c:1731: xf86ScrnToScreen: Assertion `pScrn->scrnIndex < screenInfo.numScreens' failed.
(EE) 
(EE) Backtrace:
(EE) 0: /home/pq/jhbuild/install/bin/Xorg (xorg_backtrace+0x4d) [0x55816db0dd8d]
(EE) 1: /home/pq/jhbuild/install/bin/Xorg (0x55816d9d0000+0x141199) [0x55816db11199]
(EE) 2: /lib/x86_64-linux-gnu/libc.so.6 (0x7f045aaf0000+0x43f60) [0x7f045ab33f60]
(EE) 3: /lib/x86_64-linux-gnu/libc.so.6 (gsignal+0xc7) [0x7f045ab33ed7]
(EE) 4: /lib/x86_64-linux-gnu/libc.so.6 (abort+0x121) [0x7f045ab15535]
(EE) 5: /lib/x86_64-linux-gnu/libc.so.6 (0x7f045aaf0000+0x2540f) [0x7f045ab1540f]
(EE) 6: /lib/x86_64-linux-gnu/libc.so.6 (0x7f045aaf0000+0x35012) [0x7f045ab25012]
(EE) 7: /home/pq/jhbuild/install/bin/Xorg (0x55816d9d0000+0x167854) [0x55816db37854]
(EE) 8: /home/pq/jhbuild/install/lib/xorg/modules/drivers/modesetting_drv.so (0x7f045a230000+0xb17e) [0x7f045a23b17e]
(EE) 9: /home/pq/jhbuild/install/bin/Xorg (InitOutput+0x978) [0x55816db2f048]
(EE) 10: /home/pq/jhbuild/install/bin/Xorg (0x55816d9d0000+0x76e5f) [0x55816da46e5f]
(EE) 11: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xeb) [0x7f045ab16b6b]
(EE) 12: /home/pq/jhbuild/install/bin/Xorg (_start+0x2a) [0x55816da0a25a]
(EE) 
(EE) 
Fatal server error:
(EE) Caught signal 6 (Aborted). Server aborting

Here is the relevant part of the stack trace:

#11 0x00007f045ab25012 in __GI___assert_fail (assertion=assertion@entry=0x55816dbaf958 "pScrn->scrnIndex < screenInfo.numScreens", 
    file=file@entry=0x55816dbaf898 "../hw/xfree86/common/xf86Helper.c", line=line@entry=1731, 
    function=function@entry=0x55816dbafa10 <__PRETTY_FUNCTION__.23130> "xf86ScrnToScreen") at assert.c:101
#12 0x000055816db37854 in xf86ScrnToScreen (pScrn=pScrn@entry=0x55816f903be0) at ../hw/xfree86/common/xf86Helper.c:1731
#13 0x00007f045a23b17e in try_enable_glamor (pScrn=0x55816f903be0) at ../hw/xfree86/drivers/modesetting/driver.c:771
#14 PreInit (pScrn=0x55816f903be0, flags=<optimised out>) at ../hw/xfree86/drivers/modesetting/driver.c:998
#15 0x000055816db2f048 in InitOutput (pScreenInfo=pScreenInfo@entry=0x55816dc334e0 <screenInfo>, argc=argc@entry=2, argv=argv@entry=0x7fff48ba5898)
    at ../hw/xfree86/common/xf86Init.c:474
#16 0x000055816da46e5f in dix_main (argc=2, argv=0x7fff48ba5898, envp=<optimised out>) at ../dix/main.c:191

This is a regression some time after 3340ddf3. Bisection is possible and I can probably do that later.