llvmpipe: don't tie depth clamping to clip_halfz

llvmpipe takes clip_halfz to mean D3D10 operation which now that we have lavapipe is incorrect. Vulkan allows depth clamping to be disabled and also wants clip_halfz.

This removes this workaround from the driver, however I do believe this will break the d3d10 state tracker.

Should we add a separate depth clamp state to rasterizsation to allow separate clamp/clip control? Vulkan has an extension to allow this possibility.

Orthogonal to that is if values getting written to the depth buffer should be clamped to 0,1 even if depth clamping isn't enabled. VK spec: "If the depth attachment has a fixed-point format and zf is outside of the range [0.0,1.0], it is clamped to that range before writing." However doing it just for unorm formats causes a GLES3 zink on lvp test to fail as GLES3 seems to always want clamping to 0..1 or at least I can find anywhere it asks different.

This also may point out a bug in u_blitter that doesn't set the min/max depth correctly for some states, quite when we should look at the viewport vs not I'm not sure here either.