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 <marek.olsak@amd.com> Reviewed-by:
Nicolai Hähnle <nicolai.haehnle@amd.com> Reviewed-by:
Emil Velikov <emil.velikov@collabora.com> Signed-off-by:
Adam Jackson <ajax@redhat.com>