llvmpipe: remove clamping to [0,1] for tri offset

this was added in 315f8daa without much detail, then refactored a bunch of times, and there's even now been a comment added questioning whether it's correct: it does not appear to be.

fixes lavapipe cts cases: dEQP-VK.draw.inverted_depth_ranges.depthclamp_deltaone_bias_clamp_neg dEQP-VK.draw.inverted_depth_ranges.nodepthclamp_deltaone_bias_clamp_neg