Put the frames on a display through KMS, checked by writeback

vega display draws the same frames on a display with the kernel's mode
setting and no library between: a connected connector's mode on a CRTC, its
primary plane flipped between two dumb buffers by atomic commits, the grid
as many cells as fit. Frames are drawn in cached memory and copied over,
since blending over write-combined buffers read them back slowly (a 1080p
frame took 370 ms; now 20.6 ms with a vertical sync and the check below).

This Pi has no screen but its display controller has writeback connectors:
--writeback sets a mode on one and takes every frame back as the controller
put it out. Each came back exactly as drawn, and the new check finds it the
pixel surface's frame for the same grid, pixel for pixel.

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