Description:
I use a Lenovo ThinkPad P51 with the Lenovo Workstation Dock in a 4 monitor configuration.
This model is equipped with a Quadro M2200 Mobile GPU and an Intel P630.
The full configuration is available in a blog post I made about this machine at https://raphael.doursenaud.fr/lenovo-thinkpad-p51.html#hardware-features
I also made available a copy of my display setup scripts at https://github.com/rdoursenaud/lenovo-p51-display-management

This used to work fine until the 387.12 update.

Using the internal display (via PRIME since it's connected to the Intel GPU) with or without the dock works fine.
Enabling one DisplayPort output from the dock (All outputs are seen as DP-0.1 to DP-0.3 even if the physical connectors are VGA, HDMI and DisplayPort) works.
Enabling any more than two in any configuration leads to an unresponsive, impossible to kill Xorg process and no monitor enabled except for the internal one.
Switching to a VT is also not possible.

Please find attached the logs I generated just after such a lockup via SSH.

Downgrading to 384.90 makes everything work again.

Other technical informations that may be relevant :
I'm using KMS with nvidia-modeset loaded early from the initramfs.
I'm *not* using bumblebee but PRIME.

Additional info:
* Attached nvidia-bug-report.log contains all other relevant informations.
* Maybe related to  FS#53334  since symptoms are similar but this one uses the nouveau driver.
* I also sent this bug report to linux-bugs@nvidia.com

Steps to reproduce:
Enable more than one external monitor using xrandr.