i965: Fix fallout from ARB_depth_clamp enablement that broke glDepthRange.

i965: Fix fallout from ARB_depth_clamp enablement that broke glDepthRange.

If a backwards glDepthRange was supplied (as with the old Quake no-z-clearing
hack), the hardware would have always clamped because we weren't clamping to
the min of near/far and the max of near/far.  Also, we shouldn't be clamping
to near/far at all when not in depth clamp mode (this usually didn't matter
since near/far are usually the same as the 0.0, 1.0 clamping you do for
fixed-point depth).

This should fix funny depth issues in PlaneShift, and fixes piglit
depth-clamp-range