glx apps with dri2 lock up display with xserver 1.20.1

This locks up the X display with no error in Xorg.log or dmesg when using modesetting driver with glamor:

LIBGL_DRI3_DISABLE=1 glxgears

Only cursor is visible but display becomes unresponsive. glxgears with DRI3 doesn't lead to such a problem. This also means that applications using vaapi sink (which is DRI2) don't work with glx.

I did a bisect and the bad commit is:

commit 86b2d874 Author: Daniel Stone daniels@collabora.com Date: Wed Apr 4 16:16:36 2018 +0100

glamor: Reallocate pixmap storage without modifiers if necessary

If we need a pixmap's storage to be exported to a context in which we
aren't aware of modifiers, reallocate the buffer again without
modifiers.

This makes it possible to run a compositing manager on an old GLX/EGL
stack on top of an X server which allocates internal buffer storage
using exotic modifiers from modifier-aware GBM/EGL/KMS.

Using Intel DDX driver works. Disabling glamor also works. GBM_BO_WITH_MODIFIERS is 1.