Using your patch at graphics/drm-61-kmod port and it builded fine and run is ok too.
Also, there is a pull at https://github.com/freebsd/drm-kmod/pull/348

In D49892#1137884, @dougm wrote:

I've attached a patch to the drm-kmod sources that I expect to address the problem. A similar patch may well address the other cases.

This change is part of an effort to eliminate from the implementation the doubly-linked list that vm_page_next() depends on.

{F114969018}