drisw: Enable flush control for llvmpipe and softpipe

Hilariously this is a fairly big win.  Neil's multi-context-test
improves from ~24 to ~36 fps with llvmpipe on a Core i5-3317U.  softpipe
also improves, from about 2.25 to 3.09 fps (when it's that slow, you're
allowed to be that precise).

I'd have added it to swrast classic, but the testcase wants GL 3.0 and
shaders, and that's not a thing classic has, so I figured making it work
on softpipe was crime enough.

Reviewed-by: Marek Olšák <[email protected]>
Reviewed-by: Nicolai Hähnle <[email protected]>
Reviewed-by: Emil Velikov <[email protected]>
Signed-off-by: Adam Jackson <[email protected]>

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