i965: add functional changes for AMD_depth_clamp_separate

i965: add functional changes for AMD_depth_clamp_separate

Gen >= 9 have ability to control clamping of depth values separately at
near and far plane.

z_w is clamped to the range [min(n,f), 0] if clamping at near plane is
enabled, [0, max(n,f)] if clamping at far plane is enabled and [min(n,f)
max(n,f)] if clamping at both plane is enabled.

v2: 1) Use better coding style (Ian Romanick)

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>