virtio-gpu DRM driver (2D KMS) for drm-kmod [PR #499]

Hi,

I've submitted a port of Linux's virtio-gpu DRM driver (2D only, no
virgl) to drm-kmod: https://github.com/freebsd/drm-kmod/pull/499

It gives FreeBSD guests under QEMU/UTM/libvirt a KMS path:
/dev/dri/card0, vt(4) console via vt_drmfb, and wlroots compositors
(sway confirmed, WLR_RENDERER=pixman). Developed and tested on 15.1
aarch64 under UTM/QEMU-hvf; compiles on amd64 as well.

Structure: a verbatim Linux v6.12 import (+36 lines under
#ifdef __FreeBSD__ across two files), a module-local Linux-virtio shim
over virtqueue(9), and a native newbus attach driver. Everything
shipped is MIT, including a reimplementation of drm_gem_shmem_helper —
the GPL-2.0-only file that stalled the previous attempt (#119).

Review would be much appreciated.

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