llvmpipe: rename bins to scene

llvmpipe: rename bins to scene

It was pretty confusing having an entity named "bin" and another named
"bins", not least because sometimes there was a need to talk about >1
of the "bins" objects, which couldn't be pluralized any further...

Scene is a term used in a bunch of places to talk about what a binner
operates on, so it's a decent choice here.