gallium: caps cleanups

Emma Anholt requested to merge anholt/mesa:cap-cleanups into master

Deletes redundant code from tons of drivers (leaving nouveau, because IIRC imirkin liked having it) thanks to u_screen.c, fixes up ALLOW_MAPPED_BUFFERS on software rasterizers, and then garbage collects a pipe cap that's now dead.

Check my changes to your driver carefully, my eyes are pretty glazed over at this point.

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