i965/gen6+: enable EXT_polygon_offset_clamp

Replace the hard-coded 0's with the context clamp value.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

docs/relnotes/10.5.0.html

@@ -47,6 +47,7 @@ Note: some of the new features are only available with certain drivers.
 <li>GL_ARB_framebuffer_sRGB on freedreno</li>
 <li>GL_ARB_texture_rg on freedreno</li>
 <li>GL_EXT_packed_float on freedreno</li>
+<li>GL_EXT_polygon_offset_clamp on i965</li>
 <li>GL_EXT_texture_shared_exponent on freedreno</li>
 <li>GL_EXT_texture_snorm on freedreno</li>
 </ul>