iris: add support for INTEL_conservative_rasterization

this hooks up the iris gallium driver to existing mesa bits which handle
the implementation

resolves kwg/mesa#8

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

docs/relnotes/19.1.0.html

@@ -60,6 +60,7 @@ TBD.
 <li>VK_KHR_shader_atomic_int64 on Intel.</li>
 <li>VK_EXT_descriptor_indexing on Intel.</li>
 <li>VK_KHR_shader_float16_int8 on Intel and RADV.</li>
+<li>GL_INTEL_conservative_rasterization on iris.</li>
 </ul>
 
 <h2>Bug fixes</h2>