freedreno/a6xx: Add ARB_depth_clamp and separate clamp support.
Passes piglit depth_clamp, depth-clamp-range, amd_depth_clamp_separate_range. This is part of enabling GL 3.2 (the other is bumping PIPE_CAP_GLSL_FEATURE_LEVEL, which I'm hoping to do once we have the KHR-GL* testing in place).