llvmpipe: fix depth clamping wrt reversed near/far values

llvmpipe: fix depth clamping wrt reversed near/far values

This wasn't handled before (the result was that no matter what value got
clamped, it always ended up as the near value in this case) (if clamping
actually happened).
Fix this by using the util helper for that (the math is otherwise "mostly"
the same, mostly because there could actually be differences due to float
rounding, but I don't even know which one would be more correct).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>