So, DRI support on my NetBSD netbook (IdeaPad S10e with xf86-video-intel) has been broken for a while. Having upgraded packages approximately at the same time, I didn't remember that I had changed Xorg's configuration to support multi-head configurations when so desired. Like me, to be ready in case your 1024x576 LCD panel might sit next to a 1280x1024 screen, you might have added the following entry to xorg.conf:
Virtual 2304 1024(in the display subsection, as seen here [1])
Unfortunately, it is exactly this size that is then used as the "frame buffer width": removing the line restored XVideo support on my smaller laptop, while restoring "i915drm* at vga?" in my kernel configuration even fixed accelerated 3D rendering. Finally!
[1] https://bugs.launchpad.net/ubuntu/+source/mesa/+bug/146859
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