[REGRESSION] 4.18-rc2: X61s thinkpad display unusable after xlock & lid close

[Vito Caputo]

Hello,

Beginning with 4.18, when I lock my X server using the `xlock` command,
and close the lid, upon reopening the lid I am not presented with the
xlock UI.

The system is not hung.  If I blindly enter the password, I get an
intact and functional pointer but none of the desktop is displayed.  I
can move the pointer, and cycling window focus generates some random
noise occasionally, but it's all nonsensical.  No windows are
discernable, it's just blackness with ephemeral noise on window cycles,
and a movable pointer.

The system does not suspend on lid close, with "HandleLidSwitch=ignore"
in logind.conf.  So this is a bit odd, since I don't observe this when I
just run `xlock` but don't close/open the lid before unlocking.

This is on Debian 9.4 amd64, the hardware is a 1.8Ghz X61s ThinkPad,
kernel config attached.  I'm using the modesetting Xorg driver on i915
KMS.

Regards,
Vito Caputo