Modesetting driver: External monitor ("reverse PRIME", Output Sink) updates very slowly (1fps) when laptop screen is off

This is basically xorg/driver/xf86-video-intel#133 for the modesetting driver: once I turn off my internal laptop screen, the external screen only updates at a very low rate, and the system becomes basically unusable.

This is a regression in the sense that before today's system update (updating xserver-xorg-core from 2:1.20.4-1 to 2:1.20.6-1, but also upgrading Qt from 5.11.3+dfsg1-4 to 5.12.5+dfsg-2 and Kwin from 4:5.14.5-1 to 4:5.14.5-1+b1 [this is just a dependency soname bump] and many other things), everything worked fine. But I think the update also changed which drivers Xorg is picking for which card -- though that is hard to determine after the fact.

Environment:
Debian Testing
xserver-xorg-core 2:1.20.6-1
Intel Corporation HD Graphics P530 (rev 06)
NVIDIA Corporation GM107GLM [Quadro M2000M] (rev a2)